Parquet Floor

Our oak parquet floor in hall is worn in places and many gaps due an earlier flood (20 years ago) I sanded it down and filled gaps with filler to match (or nearest) and put 6 coats of polyurethane varnish on it. 12 years later it needs re-doing many bits of filler have come out. Wood is oak about 20mm thick. Area about 10 m2
Thinking of buying a belt sander and doing 2 m2 at a time, mixing sawdust with resin to get a better adhesion in gaps. And polyurethane varnish again.
Any suggestion re size of sander, grit, etc Comments?
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