I guess the idea is: how much can we cut from the game such that what is left behind still teaches people the fundamentals. The idea would be for graduates of "FAF lite" to understand the most important aspects of the game to where it's not that hard to learn whatever else they need to learn. And not everyone might agree with me on this, but I think the most important things to learn are how to spend mass, balance mass/power, make units and use units. All of which you can learn at the T1 phase.
I would suggest removing the mex upgrades entirely. That is what I did for "stone age." There really isn't a point in teaching people the "wrong" balance for when to get mex upgrades by making them more expensive. And as long as they are an option, some people will try to get them, which will mean very slow, turtle-y games. If the point is to simplify the game, removing the ability to spend resources on growing your economy will definitely make it simpler.
Learning how to balance resources between making units and making mex upgrades is just not something that new players need to learn. Mostly they just need to be encouraged to get spam going and learn how to use it. They should be focused on spending all of their resources on making factories and keeping their bars balanced and using units efficiently. If they learn all that, it will be no trouble at all for them to graduate to "real" FAF and learn about mex upgrades. Everybody basically comes in knowing that they want to have upgraded mexes. But to really teach someone how to balance between spending mass on factories/units and spending mass on mex upgrades, they would be able to learn that balance much better if they already understand the value of units. If they know what 900 mass worth of tanks can accomplish in the game, they will be in a much better position to make a decision about whether upgrading a mex is smart.
And if you want to focus on fundamentals without complex stuff, I would suggest getting rid of T2/T3/T4 entirely, like Stone Age. People can learn about balancing their mass vs. energy income while they're only using T1 tech. They can learn about unit movement and unit composition and intel and shields with only using T1 tech. They don't need cloaking and stealth and jamming and TMD and SMD and long-range artillery to learn the basics of the game.
People might eventually get bored with the low-tech stuff, but not really. High-level players draw 5x5 maps on the ladder and there's nothing boring or dumb about those matches. A person could play t1-only matches for a very long time before they got bored with the game. And obviously once they get bored they can play full-version matches.
I'm not sure about the idea of getting rid of reclaim because it ends up being so important to the full game. If you teach someone how to play really well in an environment where there is no reclaim, and then they graduate to the real game, maybe they can pick it up quickly (after all, if they understand the value of mass they should understand the value of reclaiming it). So I can't say that removing reclaim (or at least, removing or reducing the value of unit wrecks) would be harmful to people learning the game. Removing the "mass donation" aspect of the game could encourage people to be more aggressive, which is usually what we want newer players to do. I think it's normal for newer players (not all of them) to be overly passive.
If you want to avoid overwhelming people, just don't use big maps, navy maps, or any kind of transport rush map at all. So obviously you need to put thought into map selection, just making a mod alone isn't going to be enough.
Perhaps even get rid of transports entirely. Drops obviously are very important in the full game but you could still learn a lot about the fundamentals of getting your economy up and moving units around without needing to teach people about using and countering drops.
Getting rid of air units entirely -- I would be against that.
I can't think of any T1 units that should be eliminated. The only ones that could be justified are LABs. In theory you could still teach 99% of what people need to know about raiding and defending without people having LABs available. IF you get rid of LABs, I think you have to nerf the selen, like reduce its DPS and cost.
Get rid of veterancy: that seems like a good idea. If you want people to focus on the basics, having an ACU that magically gets hit points back suddenly doesn't seem important. When people graduate to the "full" game, it would be very very easy to learn about veterancy bonuses and their importance. You don't need 120 IQ to understand that suddenly getting an extra 1,000 hit points on your ACU during the middle of a battle is very important/valuable.
Faction choices: you can learn the game fundamentals without aeon at all. Same goes for Sera. You could just eliminate those two factions entirely. I don't think it would be hard to re-code the Aeon ACU to actually be a UEF ACU and to recode the Sera ACU to actually be Cybran (meaning it has the same look/statistics/abilities as that faction's ACU, so if you pick "Aeon" you spawn in with UAL0001 which is identical in appearance and function to UEL0001, meaning you can only make UEF factories and get UEF upgrades and the ACU looks like a UEF).