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It's been a while since I was building but I think I was 80 grit for hard woods and 120 for softer woods and that worked pretty OK with a Performax 10 - 20.

Someone back then ordered some 36 grit and could not use it, obviously so they sent it to me. I played with it and it was like a garbage disposal for wood. Put the wood in one side and nothing was left to come out the other side. An exaggeration of course but 36 grit was way too abrasive for our purposes.

Regarding deep scratches a huge part of building for me was learning where doing something at one end was going to create much more effort at the other end and thickness sander scratches are that kind of thing.

(My minimal experience)
I would just like to reiterate, the 60 grit that was recommended to me was for hardwood. This did work. Small pass increments. Yes, more scratches to sand out.

One thing i also did when i first got my 16-32 was remove belt, place a straight edge on bed, drop drum down to straight edge while shining a light from behind.
I biased the open end with feeler gauge and then tightened everything up. It made sense to me and it worked.....but maybe i just got lucky. : )

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I took the sandpaper off and loosened the adjustments on the table and slowly moved it until a board pushed into the drum, entered the exact same distance on each end and the middle. I made marks on the table with a marker to measure. The drum was positioned at a height that was just barely too low for the board to clear. I was essentially using the board as a feeler gauge. I got the exact same measurements in all three places, so I know my table is flat.