I'm building an encounter for a level 20 5e one-shot, where the party is told to pick a single combatant to represent them. The idea then is that I'll select a different monster depending on who they pick. I'm having trouble finding good opponents for most of them. I want them to be CR 9 (8 or 10 if necessary), and pose a challenge for that specific PC. Any suggestions on where to look?
Alternatively, I'm considering just using a Fomorian across the board. Thoughts?
I'll post minimal PC builds below. I can add info as needed:

Half-Elf, 6 Warlock (Hexblade), 14 Sorcerer (Draconic).
Str 8, Dex 20, Con 16, Int 10, Wis 8, Cha 20

Lizardfolk, 15 Ranger (Hunter), 5 Druid (Spores).
Str 11, Dex 20, Con 18, Int 7, Wis 16, Cha 6

Half-Elf, 20 Rogue (Assassin).
Str 10, Dex 18, Con 12, Int 15, Wis 14, Cha 12

Half-Elf, 7 Fighter (Arcane Archer), 7 Bard (Whispers), 6 Warlock (Hexblade).
Str 8, Dex 14, Con 12, Int 18, Wis 12, Cha 18

Warforged, 15 Fighter (Battle Master), 3 Rogue (Mastermind), 2 Paladin.
Str 8, Dex 20, Con 16, Int 10, Wis 8, Cha 20

Race TBD, 20 Wizard (Necromancy).
I don't have much on this PC yet, but I expect them to be minion-based