True polymorph says:
Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
I assume it changes the creature's type as well.
Magic jar says:
Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell's material component. While your soul inhabits the container, you are aware of your surroundings as if you were in the container's space. You can't move or use reactions. The only action you can take is to project your soul up to 100 feet out of the container, either returning to your living body (and ending the spell) or attempting to possess a humanoid's body.
Is it possible to true polymorph a non-humanoid creature, wait for an hour to make changes permanent (so one doesn't break concentration), use magic jar, possess target's body and then dispel true polymorph with dispel magic thus gaining all the perks of original host's body?
I was shown it might be X-Y Problem and advised to add a little of background information.
As for now my PC has decent mental abilities but his physical attributes leave much to be desired so I was looking for a way to improve them (on par with AC and HP pool) without removing ability to cast spells and leaving his mental attributes untouched. Polymorphing into a dragon is still a thing though it does cap at CR20 Ancient Brass Dragon.