Should I sand cured Deep Pour Epoxy before the next layer or flood coat?

Only if it has cured past the recoat window.

UltraClear Deep Pour Epoxy is built up in layers of up to 1 inch, poured every 12 to 24 hours. If more than 3 days have passed since the previous Deep Pour layer, lightly sand-scuff the cured surface with 220-320 grit sandpaper, wipe the dust off with a clean rag and denatured alcohol or acetone, and pour the next layer directly over it. Keep any new Deep Pour layer at least 1/4 inch deep; it can't be poured thinner than that. The same applies to a Deep Pour that has been sitting for weeks or months: scuff, clean, and pour.

When you're ready for the top coat, you don't need a seal coat over cured epoxy. Seal coats are only for wood and other porous surfaces. If the Deep Pour has cured past its recoat window, scuff and clean it the same way, then pour the UltraClear Bar & Table Top Epoxy flood coat directly over it. The new coat fills in the sanding scratches and cures with a clear, glass-like finish, so the cloudy look from sanding disappears.

The full process for combining the two products is covered step by step in the river table guide: https://ultraclear.helpscoutdocs.com/article/859-table-top-epoxy-deep-pour-epoxy-river-table