dave2008
Legend
People have taken the idea of "action economy" way too far. It's important to understand how much of a force multiplier it is; but the multiplier, while large, is finite, and the DM's resources are infinite. Sufficiently high numbers can counter any number of actions and absorb any amount of DPR.
Can your PCs dish out 200 points of damage a round? Give the monster 1000 hit points. Can they apply three save-or-lose effects per round at DC 19? Give the monster +10 in the relevant saving throws and 6 shots of Legendary Resistance. On offense, you don't have to keep piling on legendary actions, just crank up the damage on the actions the monster already has, until it's dishing out roughly one PC worth of hit points every round. Once you get a feel for what your party can do, you know what it takes to give them a fight; you just have to be willing to do it.
This knowledge brought to you by the broken bodies of PCs who went up against solo monsters in my campaigns.
I'mm not sure what you mean. The point of a revised action economy for solos is that it is more fun. I, and my group, find it more exciting if the dragon can act throughout the round instead of just once. For us, it isn't about difficulty, but enjoyment.
Also, it is not fun (for us) to simply make the monsters strength react to the abilities of group. Now, I have done that at times, but it generally breaks our immersion if we I treat monsters that way.