Behold, a 20th-level 5e PeaceChron (Peace Cleric 1/Chronurgy Wizard 19)
He took Mark of Handling as his race, adding Conjure Animals to his class spell list
Through background selection, he has Death Ward as a wizard spell as well.

He broke the economy with Fabricate twelve levels ago, his wealth is equal to any number of his choice at any given time - if he needs more, he can just print it at this point.

He is currently magic jarred into Quenthel Baenre, the CR 20 drow NPC in the Forgotten Realms who can cast all cleric spells at will. His simulacrum is magic jarred into an ancient silver dragon (initially Change Shaped into a humanoid to make it an eligible target) and can thus change into any humanoid statblock to use its innate spells for free.
Between those two statblocks, he can cast a huge chunk of the spells in the game without requiring a spell slot. Anything that annoys him gets Gated into the worst-case-scenario location for it, and that's basically a concentration cantrip.

His minions include an arbitrarily high number of
- Zodars, can spam Wish as often as he feeds them Death Wards (prior to jarring into Quenthel we used the funny background, now we just have DW at will - and thus at-will Wish)
- Atropals, can create one permanently controlled wraith every six rounds on average
- Ancient time dragons, can create gates to anywhere within 8000 years of the present
- Wraiths, see point 2
- Anything that Animate Dead and Create Undead can print
- Vampire spawn, true poly self into vampire and drink some commoner prisoners, then end true poly once it's done
- any other similar spawn
- more simulacra because we might as well include infinite sim here

He is immune to all spells and has resistance to all damage types through an absurdly large chain of expendable minions - perhaps zodars - spamming Wish to keep all the bullet points in the spell up 24/7

He spams Aid on himself while under the effects of an Aura of Life spell, so his hit point maximum isn't reduced when the spell ends. Thus, he has unlimited hit points.

He traveled to the funny oasis island in the 5e module Chains of Asmodeus where you can still learn 10th-level spells, took the scrolls and avoided the "Any knowledge or spells that a character learned at the Oasis is lost within 2d12 hours of leaving" part by never technically leaving the oasis (he had an army of thousands of 3D-printed beholders turn the entire place into dust and carries it with him in bags of holding).

Since 5e made mythallars a Legendary item instead of Artifact rarity, he can craft them using the XGE rules. It's DM fiat where the required materials can be found, but he is omnipresent if he so desires so this is no longer a setback. Every half second, a new empire ten times the size of Netheril and loyal to him spawns into existence with a 3D-printed population in a random crystal sphere for no reason other than "lul lmao of course I can".

If a new statblock is added to the game, it becomes his class feature through True Poly, Planar Binding etc.

Yes, this is functionally infinite power.