Looks excellent. I would also allow an attribute upgrade for an expensive XP price, and attribute modifier for skills maybe.
Viewing post in OSR D&D Without classes comments
I'm personally not inclined to introduce attribute upgrades or detailed skill systems in Old School D&D.
I would probably place ability score improvements at 500 and cap it at a total maximum of a total 10 additional stat points across al six stats.
For skills, I'd probably add another degree of specialization instead of ranks. This would translate in double Advantage, roll three dice and pick the best. While I don't cap skills, I'd probably limit the number of a character's specializations at 6.