Ply over glued plank subfloor questions (with pictures)

Hello all, I appreciate any help or advice. I recently acquired a raised foundation shell house that is mostly complete but is in serious need of flooring. As you can see in the pictures the subfloor planks are not a usual width and are narrow and long. There are gaps which seem large enough to have me seek advice before putting carpet or linoleum over it.

due to the narrowness of the planks and the numerous gaps/edges I want to lay some 5/8ths ply to strengthen, insulate and provide a more consistent surface. A few spots you can see light through the floor and right to the foundation. I am conflicted on how to approach the prep. I was going to lay the ply perpendicular and set with nails to the planks avoiding the joists.
Should I nail all the planks or just leave then glued as with no nails? I will have to pull some to find the joists.
Also for the parts near the walls should I use gap filler and sand it level or just lay the ply over the gaps?
I was recommended to leave 1/8th gap between ply and the wall is this the correct amount? Once i put the baseboards in that should be covered but I want the house to be well insulated as it gets hot and AC is expensive so the smallest gap that wont cause issues is preferred.

If someone contracted you to install a floor what would you do?

I b b . c o
https://*******/dHe5wJ
https://*******/g15pOy
https://*******/n2BUOy
https://*******/k2uNbJ
https://*******/e1ZzOy
https://*******/gsuNbJ