I suspect that all the OP really NEEDS to do is ride this bike a bit more and loosen up dried or hardened grease. Adding a drop of oil, or solvent at the gap, and spinning the cassette, to draw it in, and circulate it will also help.

A better, more thorough, flush, rinse and lube sequence might be a good thing, but probably isn't necessary.

BITD, flushing old or clogged freewheels was easy enough to do off the wheel, but freehubs are harder to remove (depends on the design), so flushing is often done on the wheel and that can be messy.