So, it's been 2 years since the last time I was playing and did some experiments with Trade Routes in order to learn how they work, so I figure it's time for an update :)

As far as I know the info I worked out in that previous post still applies, but the new Economy/Trade Scan view no longer shows any Trade Value numbers for individual systems, just the total per Trade Company HQ, so if anything the Trade Routes system is even more obscure than it used to be...

I was curious how Luxury income for Trade Routes worked, but unfortunately much like 2 years ago was not able to find any fully up-to-date / accurate info online.  Old posts like this one, or this one are fairly good and give a nice overview, but still contain a few errors or omissions.  

I spent a few hours playing around with Trade Routes last night, and here's what I've found (Disclaimer that I am using the "Enhanced Space 2" mod, but to my knowledge this does not affect how Trade Routes function).

• From my testing Trade Luxury income is totally independent from System Population, despite the other forum post formula claiming it scales with population (perhaps this has since been changed in a patch?).  It does scale linearly with System Level though.  It also scales with trade route length.  My rough guess is that the actual formula used for Luxury income is something like:

Base X Luxury Income = System Level * 0.01 * System X Luxury Production

• Unlike what the reddit post implies, + System Trade Value % bonuses work on both Dust & Luxury income.  Putting +% System Trade Value improvements on the System which is producing the Luxury will increase that Luxury income accordingly.  
• You get a small amount of Luxury income for Luxuries on systems adjacent to HQ / Subsidiary (even when totally uncolonised.)
• Adjacent Outposts, if the outpost planet gives Luxuries, give more Luxury income than an uncolonised system, similar to a Level 1 Colony
• Luxury income is based on final luxury yield from the system producing the Luxury, factoring in +% Resource yield bonuses from heroes, etc., not just the base deposit size
• Systems adjacent to HQ / subsidiary systems seem to give approximately 1/3 of the Luxury income compared to an equivalent Luxury production on a system that actually has the HQ / subsidiary.

So to maximise your Luxury income from Trade Routes, you want in rough order of importance:

• High Luxury yields on the systems with Trading HQ / subsidiary (big Luxury deposits, Heroes with +% Luxury deposit yields, etc.)
• High Luxury yields on systems adjacent to the Trading HQ / subsidiary (these only worth approx. 1/3 as much though).  Outposts / uncolonised systems still give a little bit of Luxury income.
• High System Level for the systems producing Luxuries, but only if they are on systems containing or adjacent to a Trade HQ / subsidiary (adjacent systems only worth approx. 1/3 as much).
• High + System Trade Value % bonuses on the Systems producing Luxuries, but only if they are on systems containing or adjacent to a Trade HQ / subsidiary (adjacent systems only worth approx. 1/3 as much).
• Long length between Trade HQ and subsidiary
• High Trade Company HQ level.  I think this increases faster the more Dust it produces, so perhaps having it earn more dust through investing in some Bullion Freighters would be a good idea early on?  
• I suppose investing in the +Luxury freighters for the Trade company as well, but they seem to give very small bonuses.

As far as know, other factors like System population, Luxury producing systems not on/adjacent to HQs/subsidiaries, etc. all don't matter for Luxury income.  Hopefully this is useful for other people like me who wanted an idea of how this worked so they could make more informed choices about creating / developing Trade Routes.

Of course, given how little documentation there is for this stuff, I could still be mistaken / missing something, so if anybody know some more stuff about how Trade Routes work, I'd love to learn!