Mystic adepts are just a tough nut to crack because their entire gimmick is giving you access to basically the whole buffet of magical options, so limiting their choices sorta goes against that, but if you force a literal divide of resources they become too weak.
5e maybe was on the right track in terms of how they handled their adept side, mainly that Mysads in 5e didn't get a major benefit adepts got by raising magic. But it didn't work because they didn't lose anything relevant from their mage side (Astral projection, while cool as heck, is sorta a joke because... spirits exist and are so overwhelmingly better than a projecting mage in every way that astrally projecting not only isn't meaningful to lose, but astral projecting is almost always actively a bad idea to do in any situation) and the mage side was the stronger side, to the point where despite mysads obviously being 'stronger' than mages, the mere at gen opportunity cost of being a mystic adept was big enough that they didn't actually overtake mages in terms of character optimization: Spending a bunch of karma at gen on PP to get increase reflexes really was you spending 20% of your gen karma budget on a +1 bonus to dicerolls, which is good but didn't fit into high end builds, which says a LOT about how strong 5e mages were when what was essentially 12 karma for a generic +1 to everything wouldn't make the cut!
While "Buffs over nerfs" is a bit of a meme that doesn't really reflect good game design, it may be that the best way to make mystic adepts work is to not take away their options per-say but give adepts and mages something that makes the choice to give up being a 'pure' archetype not as clear cut. Adepts could stand to get a buff anyway because the entire scheme of a hyper-specialized power source resource where you can only augment a few very specific things doesn't really work with how SR PCs are made, and it would be nice for astrally active mages to be the biggest meanest things on the astral plane with lots of cool astral tricks and toys rather than being idiots who are going to get eaten by a force 4 spirit.
Either way, I don't think 'try to limit mystic adepts by removing specific aspects of magery' is a good plan because that is pretty binary: Either you have enough magic stuff to basically get all the good stuff, or you don't. Like if you only get one of enchanting, sorcery, or conjuration, then its super obvious (at least with 5e's balance between those abilities and adept powers) its crazy not worth it to be an adept ever, because sorcery+conjuration is going to be way stronger than anything you get as an adept. But that has a lot to do with how 5e handles buffs and spirits. 6e seems to have toned down buffs, but not as much spirits, and so if you did that and spirits ended up being the 'good part' of mages, then you still have mystic adepts being way better than being a mage.