Syndra's fine now, though? The only issue I might see is the Q +2 second passive giving her too much accessibility still, but it's not stuck out in my mind so far yet. You could even nerf it to 1 second and try it from there, but if it had to be nerfed again I'd just revert that passive component entirely.
Her old W passive was pure shit and had no reason to exist, so the True Damage mid/late game buff definitely helps a pain point she had in that part of the game.
The only other buff that remains is the 0.1+ AP ratio on her E and 2 second lower base cooldown. Yeah you could this nerf to trim power, it doesn't really feel all that important to me in either direction.
This whole 'miss everything, R and kill the target' spiel is more a result of environmental factors. Her direct ultimate damage has never changed--even when Thunderlord's was first introduced, it wasn't a game breaking problem for her (Literally the 6 months before the MYMU, she suddenly didn't evolve into a nightmare). But, a lot of MR has moved around post-MYMU, and squishy targets (ADC primarily) don't have the space to build defensively, so their weakness of actually getting blown apart is accentuated quite heavily. Virtually every midlaner/jungler that builds to deal with her, still deals with her as well as they did before.
The other big factor is Morellonomicon is a grotesquely overpowered item. Many midlaners who rushed Athene's, of which Syndra was one, often had +60 AP and +MR on their laning phase. So, their net damage wasn't actually all that high, as it was a modest AP amount on top of built in tankiness. Take that away, now add Morello's with +100 AP for nearly the same price. That's a whole extra large rod's worth of damage, on top of the removal of ambient MR, enhancing their first buy power. That item singlehandedly threw out years of relatively stable mid laner damage with how good it is.