The hallway pic shows the pine subfloor refinished in a small bedroom and unfinished in the hall. The living room pics show where I removed a single piece of 1/2” plywood to expose the painted pine subfloor.
The pine subfloor looks great in the small bedroom, but it does have larger gaps between the planks. I think wood filler would crack and fall through the cracks every few seasons.
Looking for suggestions to choose between leaving / or removing the plywood and installing prefinished red oak flooring on top ply or subfloor, or removing all the ply and refinishing the pine.
I have a little over 1000 sq ft of flooring to work on between living room, family room, kitchen, master bedroom and hallways.
Thank you!

Not sure if I have or will have permission to attach the photos. Will post them when able.

If the pine floor was meant to be a finished floor instead of being covered with wall to wall carpet or linoleum and was the best quality it would be vertical grain flooring. Plain sawn is not as good as a finished floor. Keeping the original flooring to the house would be a great choice. Sometimes the flooring has never been sanded with a machine. Removing the paint is a problem because it might have lead in the paint. This makes for extra work to not contaminate the house if you are living in it at the time. You will need to clean up very well after sanding and before the final coat of finish. Is the flooring visible from underneath? When working on old floors that have been sanded and are thin in doorways or have gaps between boards it helps to add blocking between the floor joists if there is only one layer of flooring. This will help stiffen the flooring so the wood patch used to fill the gaps will not crack out and be vacuumed away over time. It takes a lot of work to refinish a painted floor.