I think something that could help this reductivity a lot is an eximus redesign. The only ones I find meaningful are the infested ancient eximi, because they have a little more staying power than the others. Grineer and corpus eximi, though, die too fast, and as a whole they're not dangerous enough. I don't think they should be super high-health minibosses, though; that would go against the current feel of combat, which I like.
Instead, give eximi some attacks which:
present a serious threat to any player, maybe by doing straight health damage, or disabling abilities, or some other effect which is difficult to tank
are easily visible and slow moving, like the flame eximus wave
high cooldown, but trigger early, so they can be relevant before getting annihilated by plasmor fire.
Also, bump their size scaling up another notch, they aren't quite b i g enough.
I think in the end they should be more like nulliifiers, the only enemy in Warframe that I feel really requires the player to change pace to deal with. They should still be vulnerable to high damage weapons though, as every other enemy in the game seems to be drawing players towards lots of bullets or AoE damage, and that adds to the reduction. The best performing weapons in the game are more often just those that hit multiple targets, not those that would be statistically better.
Also, other "high ranking muckity-mucks" still feel too fragile and undifferentiated from other enemies. There are some enemies which feel distinguished from combat, mostly nullifiers, heavy gunners, bursas, and noxes, all of which I really enjoy. Others I can tell apart visually, like scrambuses, infested enemies, and ghouls, but in the end are just another bowling pin for whatever plasma ball I'm throwing. Bombards and napalms and such sometimes catch my attention by blasting me with a missile but usually aren't that interesting. In the end, I think Warframe just needs more important targets, ones that are worth paying attention to and taking out before the others. Once again, I don't think this means giving them higher EHP, because that's just not the way this game feels. Instead, give them visual distinction and slow moving, dodgable attacks that can stand out among all the bright energy colors, so that we can easily notice and direct attention to them for a split second.