I'm not quite sure how to decide how many techs to research or skip in ES2.

In ES1, each tech had a fixed cost, and advanced techs cost exponentially more than less-advanced ones.  This meant that you sometimes would rush ahead to get an important tech, but eventually you should get just about everything, because even a bad tech is probably worth getting when it only costs a small fraction of your other options.  The order of research might vary, but in the long run I expected to get basically everything.

In Endless Legend, eras are unlocked based on the total number of techs you have, without regard to what era they're from; you could hypothetically unlock era 4 without researching a single tech from era 3.  More advanced techs cost somewhat more, but the main component of cost was the total number of techs you already had; there's no situation where you're choosing between 1 advanced tech or 3 earlier techs for the same long-term cost.  So I basically ignored the era a tech was from and researched whichever tech seemed to provide the greatest benefit, with the expectation that there would be many techs I never got at all.

Now, in ES2, every tech you get increases the cost of all future techs, but not not as much as in EL, and advanced techs cost exponentially more than earlier techs like in ES1.  Also, unlocking the next tier is based only on the immediately-previous tier, rather than the sum of all previous tiers, so researching extra early techs increases your costs without contributing towards unlocking a tier.

So if you want to unlock a given tier as quickly as possible, you need to research only the minimum number of techs from every previous tier.  From this perspective, every extra tech is wasted science for a tech that doesn't help unlock anything, PLUS it makes all the techs you actually need even more expensive!

On the other hand, if you look at only the immediate benefits of a given tech, earlier techs seem to often be more cost-effective, because they cost dramatically less science.  Additionally, if you research the bare minimum number of techs, you're going to have some pretty big holes in some areas (like only being able to unlock one of the two tier 1 strategic resources, or having a bunch of planet types you can never colonize).

So I'm having a hard time conceptualizing the true long-term benefits of getting or not-getting extra low-tier techs.  I'm not even sure if the design is intended to push me in a particular direction or not.

Anyone have a good rule-of-thumb they use to decide how many techs to research from a given tier?  Minimum to unlock the next tier?  All of them?  Somewhere in between?  Does it change depending on the tier or the stage of the game?

(Obviously, even if you're planning to get more techs from an early tier, there will be situations where you research a specific advanced tech first and then come back to the cheaper techs later.  I'm not asking what order to research them in, I'm asking how many you expect to research eventually.)

Thanks for all the detailed info!  A somewhat related question I have:

Is it often / ever a good idea to research a tech from a "higher" tier using the Science Party's "Oracle of Science" forced law?  I'm playing with a Science based faction for the first time, and so far haven't bothered to take advantage of it yet.  It always seems like I'm in a position where I can choose to research one of the "higher" techs using this law, but it will require ~12 turns to research, vs grabbing a tech on an unlocked Tier in just 3 or 4 turns.  And in most cases, whatever reward is given by that ~12 turn tech is something my empire is not yet equipped to make use of, such as a building or ship which requires 2,000 Industry while all my Systems only produce 100~200 Industry.

Is just going through the tech tree in a spiral fashion, unlocking T2 on all trees, then unlocking T3 on all trees, then unlocking T4 on all trees etc. the typical way to go?  That's what I've felt drawn to do.

The main use of Oracle of Science is skipping past the fourth ring in order to get a science victory faster. There are a few other cases where I'd use it: the main two are skipping the second ring in the top quadrant for the same purpose if I'm lucky enough not to have needed any of those techs to defend myself from my neighbors and skipping a tech or two in the third ring of the left quadrant when going for a wonder victory. In general, though, it's just as you say: it's a lot of science to spend just to unlock something that you're not ready to build. There are a few fourth-ring techs that might be worth it (the one that adds two trade company subsidiaries or a shipbound affinity tech that unlocks better ark modules, for instance), but the factions that want them are not usually going to be running Oracle of Science.

As to the spiral pattern, it's not a bad choice, but there are better ones. Outside of multiplayer, a "just-in-time" approach works very well for military technologies. The AI gives you a fair amount of warning before it attacks, so you can often delay some of the top-quadrant techs until you get that warning and then research what you need to upgrade your ships and combine them into large fleets in just a few turns. This is an especially useful thing to do in the early game because some of the rings in the top quadrant make you build more experienced ships when you unlock them. Each level of experience gives your ships more health, but makes them cost more industry to build so when you're only fighting pirates, it can be better to save that industry (especially on exploration and colony ships, which of course have little need for health).

Even outside the top quadrant, there are times that you might want to research a high-priority third-ring tech before researching some second-ring techs, or a fourth-ring before third. Some examples are the tech that unlocks trade companies if you're going for trade routes (since the faster you get your first company going, the faster you'll unlock the second, level them both up and start making real money), the aforementioned +2 subsidiaries tech for similar reasons, the technology that unlocks wormholes (if you don't have good systems to expand to and so really need to get out of your home constellation) and any tech that makes your ships stronger or increases the fleet cp cap if you have an important fight coming up.

Finally, it isn't a terrible idea to delay Plasma Metallurgy until you have a planet with Hyperium or enough population on planets with strategic resources to make its system improvement worth building. Researching it before then just increases the cost of every tech that you research for no real benefit, and it's cheap enough that you can pick it up in a single turn once you do need it.

Thanks for all the details!  Wanted to make sure I wasn't missing some obvious application of the law.  And yes, that's a good point that there are few techs which give instant benefits without having to build anything, such as the CP upgrades in Military or ability to mine Strategic resources, so those might be mmore attractive for Oracle of Science (although again, I often find myself able to research 3 or more equally useful techs in the same time to unlock one of those.)

the technology that unlocks wormholes (if you don't have good systems to expand to and so really need to get out of your home constellation

Although, speaking of Wormholes, in most of the games I've played in ES2 I've never researched wormholes, since you unlock free-space movement before wormholes (opposite of ES1).  Am I missing some reason Wormholes are attractive?  It's never appeared to make any sense for me to research it, since by then I already can travel outside of star lanes, and thanks to Explorer probes I already know where all the nearby constellations are by this point, and may even have outposts/colonies in new constellations by the time Wormholes are even an option.  It usually appears to only save around 1 turns worth of travel, since wormholes consume all movement points anyway, and my fleets may have several "fleet movement upgrade" modules by that point.

Wormholes are by far one of my favorite techs!

In my current game, I can cross half of the galaxy in one turn thanks to well placed wormholes. It is particularly powerfull when another Faction does not have access to this tech and try to attack one of my core worlds using only free-space movement: It allows me to place a welcoming committee with 20 000+ power in the blink of an eye ;)