Hmm, re: control spells and the legendary resistance workaround, one way that has worked well for my campaigns (and not using legendary resistance) is to design and treat “solo” creatures as multiple separate creatures with a shared HP pool.  That way, they act multiple times per round on different initiative counts (often I will also parcel out abilities to each initiative count entity or otherwise give each of them at least slightly different but thematically linked abilities) and they also get multiple chances per round to save against effects placed upon them:
Stuns and hard control effects are applied to the creatures’ next initiative count entity, shutting it down for at least one round without becoming a complete shut down of the creature.  Which leaves this still a very worthwhile tactic for the PCs, just as it would be in a non-solo fight to shut down an individual creature out of a group*, as it knocks out out 25% or 33% of the solo’s actions for at least a round.  
Other soft controls (such as movement, weakened, etc) I generally let affect all initiative entities of the creature, but there they can attempt their saves saves at the end of every one of their turns in the init order.  And/or sometimes, depending on the creature, I will give them the ability to downgrade the effect at the end of one or each of their turns.  Again, the idea that it’s still worthwhile for the PCs to do and it hinders the creature but doesn’t completely shut everything down.  
At certain HP points one of the creatures’ initiative entities gets taken out (usually in a pre-determined and specified order), whittling down the creature and it's abilities as the encounter progresses.  It doesn't need to be an even split either;  so if a creature has 400 HP and four initiative entities, perhaps the first entity is taken out at 250 HP remaining, the second is taken out at 100 HP remaining, and the creature dies at 0 HP.  
*  Really, this whole idea is just leveraging what's been noted above, that standard combat routines work well enough outside of the edge case of a “solo” boss, and so this is keeping with those routines just rejiggered a bit to represent a single powerful entity.