If your gonna use the stat blocks suggested and modify em for Dark Lords, at least make sure they get
Maximized HP. So even if you have Ivana as a
Spy, she's not going to have just 27 HP, but 49 HP instead. (IIRC, you Maximize monster HP by multiplying the Hit Die calculation in the stat block together, right? So for the Spy that would be 6x8=The Spy's Maximized HP total of 49.)
Probably added in some appropriate class levels/features into there as well. A Dark Lord should probably be doing at least more than the basic/level 1 damage hitting Sneak Attack if they have Sneak Attack listed in there.
I'd also say no lower CR ranked NPC monster stat blocks for Dark Lords as well: So no Mage stat block for Tsieng; either go big with Archmage or go home(before the session is over home as well!)
And probably the guidelines for making an "elite" version of the character or what not would probably help out too. I like how this link goes over something like this or making a more elite version of a monster in dnd.
Now I will agree that perhaps only the super big name Dark Lords, like Strahd, Soth, Azalin Rex, and so forth should be the only ones that gets a Mythic Trait. Small fry like Ivana wouldn't have a Mythic Trait. Ironically the Duhallan stat block in Van Richten's Guide to Ravenloft, is the
only "Dark Lord" in the book that gets one. Which I found interesting. (And since that would be the stat block you would use for The Headless Horseman, it shows that yes, Dark Lords should be able to get one too.)
Want to learn how to make elite humanoids in Dungeons & Dragons Fifth Edition? Here's a guide that will help you make them quick and easy.
dmdave.com