A cabinet scraper with a good edge will be massively quicker than sanding. I recently cleaned up my >10 yr old maple Boardsmith that’s been regularly oiled and waxed. Sharp scraper produced fluffy shavings off the end grain that quickly cleaned up all the cut marks. Sanding endgrain is slow, oil and wax soaked endgrain no fun.
If you’re set on sanding, start coarse at most 80g if not coarser. The challenge with power sanders and coarse grits will be keeping it flat, a wonky top will concertina every slice. Start with crosshatching with pencil and remove evenly then hatch again- like flatting a stone. Then work up to about 240 (80-120-180-240)
Another option would be a finely set block plane, but if you already had one and could tune it well enough to shave endgrain then you wouldn’t be asking about sanding so unless you want a fun few weeks learning may not be an option.
Good luck, post pics as you go, everyone likes seeing pictures