Dungeons and Dragons

Im trying to create a DMPC who used to be a 10th level fighter in his youth but hasn't adventured in decades. I want to weaken him in a semi-realistic way, while keeping his skills.

I was thinking of lowering his base str, dex, and con scores by 2 and increasing int, wis and, cha scores by 1 or 2. On top of this, I thought on successful attack or physical ability checks id roll a d20 dc 10 check and on a fail he would gain a point of exhaustion. Possibly increasing that dc on fails. On failed attacks or physical ability checks he'd take 1d4 damage

Obviously this is going to be weird to balance but I have a sneaking suspicion my players are going to ask this former adventurer to come on one last adventure. Relive his glory days so to speak.

They are level 5, im willing to drop his level but I want him to have been a higher level adventurer when he was in his prime. I went with the battlemaster sub class and focused all my maneuvers on assisting. Specifically:

1 Commanders strike 2 Maneuvering attack 3 rally 4 tactical assessment 5 commanding presence 6 bait and switch 7 distracting strike

Any thoughts, suggestions, criticism, or anecdotes are welcome and appreciated.

Thank you for your time