i would have thought that it would be easier to do a cut-down from a COPY of the full sequence.
you have all your edit points there, so it’s easy to jump to them, and mark in / outs around them
plus you can remove transitions at will.
nevertheless, nesting shouldn’t require re-rendering unless there is a sequence miss-match
is your media a different frame size to your sequence?
that could be the issue.
aso how are you making the nests?
edit sequence A into sequence B?
(that could cause re-rendering if you have clip / sequence miss-matches)
try instead: DUPLICATE sequence A,
open that, select all, and Option C to nest selection.
nick