I think it's B) because this game always instigate you to play wide. That's evident even in score calculation, where most points rewarded for system count and pop count.

Amplitude made no universal mechanism for system to grow infinitely tall. You populate whole system and that's it. You can't build another borough, you can make few upgrade to planet slots, terraform here and there and again you reach the end.

Factions that are considered "tall" aren't played tall because it is beneficial to them. They are played tall because you are penelized for going wide ("Hello, Hissho"). I remember Vodyani started as "tall" faction in Early Access with their slow growth and essence harvest. But they had so little mechanisms to compensate for that, that they had to be buffed multiple times to remain competetive. As result, that allowed them to spam Arks and they become overpowered wide faction.

This is why I concur your proposal to give Lumeris some "tall" boon. I think granting permanent economical bonus empire wide for every outpost sold is a valid path for that. That will instigate to buy outposts everywhere early, then sell them actively to be able to grow tall. What's important here, is to make this bonus sufficient enough to cover system count deficit, i.e. make sure it should be long-term more beneficial to sell outposts, rather then develop them on your own. This bonus should also be decremental. First outpost sold should grant maximum effect and each next will yield progressively less, to prevent potential exploits like reselling same system over and over ad infinitum. The exact nature of said bonus is a subject for debate.

Second proposal: Outposts shouldn't be sold just via diplomacy, they should be sold on the market. Maybe make a simple auction where factions with access to marketplace can place their bids. I think it's a lot easier to sell them that way. Faction can buy outpost from Lumeris only if they are at peace/alliance with them (no bidding for you, Cravers). This will additionally reinforce pacifist gameplay for Lumeris, as most factions would want to buy systems, but will have to befriend Lumeris first.

Those are valid proposals. Especially the market one I like. Would Lumeris being able to buy AI generated populations (contracted workforce i.e. indentured service) would be a valid "feature".

Also "freeing planets" to be independent, grey, ai factions,(quest civs) for a cost. It would be like selling a powerful hero, that was grown purely for his value(long term investment) that bought himself a freedom and independence.

One can artificially manipulate player tendencies  e.g. with a Happiness-Overcolonization connection. 

Lumeris players, IIRC, aim for economic win right? 

Lumeris could more closely mirror economy - with a boom during times of expansion and "freshness" and recession after stagnation. 

A sumulacrum of this system would be Golden Ages(and Bland Ages) that would stimulate growth or udermine it, on separate system and empire-scale. Hossa, bessa, so on and so forth, simplified in form. 

Lumeris now can work twofold - either buying planets for themselves or selling them for others, with the second part of little function as of now. 

Why not add a factor of time to this equation?

My proposal:

Part 1: When Lumeris player colonizes a planet he will get a systemwide "mini golden age"(simulating a golden rush) bonus and after a set number of turns a systemwide "mini bland age"(simulating a stagnation) inverted malus would set in.  A proportional Empire-wide bonus or malus would be calculated based on number of systems with either golden or bland ages at any given time. Bonuses end, maluses can be ended.

Part2: One could sell this "investor-boring" planet then to get rid of a let's say 80% efficiency and not only lower Empire-wide malus(proprtional) but also give an Empire-wide bonus 

OR (one can choose, it's not mutually exlusive)

leave it as it is for one reason or another, for 80% of e.g. a 5 planet system in a strategic location is a lot.  

What gives: A lot of dynamics, more stratagems, simulation of Lumeris mentality of "everything is business".

Selling planets would be a valid technique then, but what is a must is to take care of planet selling mechanics:

Here Lumeris player would sell his planet 

A)to another faction, mano-a-mano in a diplomacy screen (as it's now). Effect: Planet changes owners. Dust [edit: and Influence] gained. Relations improved with a faction. Empire-wide bonus for X turns. Investors rejoice.

OR

B)to a market, creating an Independent Minor Civ, for a computer-generated price(depending on improvements, populations, ships in hangar, etc). Effect: Planet changes owners. Dust [edit: and Influence] gained. No effect on relations with any faction. Empire-wide bonus for X turns. Investors rejoice. Planet bought themsleves their independency and are an "Unknown White Empire".

B) would make sure, that planet would be sold if A) is not avaiable or otherwise not preferred. Something akin to that system could be used for Nakalim Temples. I still think Nakalim suck in virtually all departments and if I were to rewind time, I would have parked Cravers on Sobra, not Husk.

B1) Sheredyn affinity for -40% buyout costs.

Also take note that even giving away an outpost is still an influence drain a faction with 0 default influence output such as Lumeris naturally shies away from.