I built out something similar for certain demigods in my world. This is basically the monster template I made that any Key wielder would have, from powerful emissaries like Mammon's or an actual Demiurge. Some of these are taken directly from KSBD and others I made to give them a more 'divine' feel. A couple I imported from Godbound (a neat RPG btw) which stats out KSBD inspired demigod types
Legendary resistences, actions and lair actions in their realms (PCs gaining Keys wouldn't get these obviously)
The three highest ability scores of the creature are raised above 20 (range of 22-26). Everything else is set at a default of 18.
Wielder of the Key (1/day). The creature can innately cast plane shift. As long as they know of the plane of existence, they do not need material components. (Pretty obvious)
Uttering the Divine Word (3/day). The creature can innately cast divine word (spell save DC X). Their spellcasting ability is WIS/CHA/INT (Depends on which. A monk type like Solomon David would have Wisdom, a more mage type like Jadis or Mottom would be Intelligence, and Incubus and Gog-Agog would probably be Charisma. I don't know about what Jagganoth or Mammon's would be.)
Divine Regeneration. At the start of their turn, the creature regains 30 hit points. (to make them all beefier and longer lasting)
Lesser Magic Immunity. Unless they wish to be affected, the creature is immune to spells of 4th level or lower. They have advantage on saving throws against all other spells and magical effects. (Slightly debuffed Magic Immunity that Tiamat has in 5e, as KSBD Key holders technically aren't gods)
Divine Fury (1/day). If reduced to zero hit points, the creature can choose to enter a state of divine fury. They will regain (half of max) hit points, freed from any conditions or spell effects imposed, be able to move without provoking attacks of oppertunity and for the time period cannot have any conditions imposed upon them. After five rounds in this state the creature becomes incapacitated for 1 minute, and loses the divine fury condition. If reduced to zero hit points in this condition, the creature's form will disintegrate and be unable to be resurrected, though their Key will be intact. (This is taken from Godbound, and I liked it because it's sort of representative of the Key holder burning their god juice directly to make one last burst. Also because I love two-stage boss fights)
and as a legendary action Invocation of the Magus Key. Ranged Spell Attack: +15 to hit, range: sight, one target. Hit: 50 (10d10) (varies on key) damage. (This is representative of key holders making direct attacks with their Key's power, like Allison's Cubit Spear)
Now obviously the Key abilities can't be fully stat'd out because they allow the Demiurges to do some wild stuff (Mottom holding up her palace, Solomon moving two suns, etc) so there's a degree of flexibility you'd have to have. Also you have to build up their abilities beyond just their Key, after all each of the Demiurges are already ludicriously powerful without drawing much power from the Key (Solomon is a good example) so giving them class levels or importing more stuff from other RPGs (I imported the Strifes from Godbound as a way to represent super-martial arts like Ki Rata) is a way to show that these guys are tough fuckers on their own.
Now to make a PC out of this seems exceedingly difficult. Frankly I'd say if your campaign involved the players getting Keys, switch the system to Godbound or Exalted. It's going to be hard to model your players literally having the power of god (even partial) in a level 1-20 system.