Your plank flooring will come out great. Yes, rip your boards so there
will be different widths per run. Belt sanding with 100 grit will get
rid of the burrs and high spots, then 150 grit will remove any 100 grit
sand marks. 100 grit sanding marks will show, once clear-coated, but
150 grit will do you justice.

Don't worry about mill marks collecting dust or debris. Your boards
will likely be sawn with a bandsaw, and those marks are not deep enough
to worry about, especially after belt sanding, sealing and
clear-coating.

Allow your boards to acclimate to the room (with air conditioning)
before installing, to allow maximum shrinkage. Any cupped boards can
be ripped narrow and used on the perimeters or other
out-of-the-way/sight places. Any severely cupped boards can be
discarded from floor use. I suppose you want the boards, or planks, to
abutt snuggly against one another.

If you don't necessarily want the boards snugged tightly against one
another, then chink between them with jute rope. If after time, once
you have laid your floor, if anymore shrinkage occurs, chink more, but
try to avoid using several small strands to fill a larger gap. Even in
initial installation, always try to use an oversized rope and pack one
strand into the gap. You can pack multiple strands on top of one
another, deep into the gap, but not adjacent to one another, across the
gap. For repacking a gap, you don't usually have to pull out the
original chinking. Jute rope can be stained to match, or nearly match,
your floor color (before staining, run the rope through a flame to
removed loose "hair"). It can be clear-coated, as well. If no stain is
used, clear-coat a sample of rope to assess the color. "Clear-coated
only" will still give it a wet look. Jute rope will give and take with
any more (long term) expansion and contraction of the flooring. When
installing, you can chink with small rope, if you want the boards tight
against one another. If your planks are 8" or larger and tightly
abutted, then chinking during installation may be more adventageous,
but more difficult. Any chinking adjacent to a wall, if space is
available, leave a tag for removing later, if ever needed.

I recommend to never use foam backer rods to chink plank flooring.

Sonny