I don't think solo creatures work with 5E at all. Every time I use a boss critter, even one with lair and legendary actions, the large party just runs over them, no matter how outmatched they are by the math. The action economy just does not work at that scale.
I think, therefore, I am going to adjust my encounter philosophy: bigger, more diverse groups where the "boss" critters aren't necessarily simply tougher but are the ones capable of wrangling such groups. I am also throwing out the CR system for 5E completely: it just does not work any better than simply eyeballing it, IMO.