If it was mine I would just cut out the old subfloor with a Toe Kick saw on the outside edges and passes with a ciruler saw to cut it into managable sections. Once it's all out it's far easyer to to check for sagging, broken, insect eaten floor joist. Repair those then install new Advantec subflooring using constrution adhesive on top of the joist and ceramic coated 2-1/4 decking screws. Now you will have a 100% sturdy floor. Trying to patch, fill, or repair all that old flooring will never come out flat or solid.
A tool that makes it far easyer to take out the old subfloor is a Gutster. I bought mine at Northern Tool. Do a key word search on it.