Maybe I’m missing something. Was this room once a garage? Most garages here are a 4” concrete slab on a crushed base floating between 48-60” deep concrete footings that were poured as part of the home foundation. Even where the home has a basement - most do - there is no basement under the garage floor. If this is your config, outright strength isn’t going to matter, but cold/moisture and “slope” will. Most garages slope towards the bay door to help ice/snow melt and clean up run out. It’s slight, but it’s too much for a living space floor, and so it has to be levelled, but anything you build overtop is going to be plenty strong if it’s sitting on a concrete slab.
When I prepped our interior subfloor for prefinished hardwood, I ripped out all the tile and parquet, and bought a big tub of construction screws and did a little overkill, putting them down every 4” on every joist.
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