This is a really challenging topic to get into. I have nothing against tentacle hentai except that in practice I find myself complaining about a lot of the things you're mentioning - the animation just never looks natural. This is probably one of the best submissions to deconstruct, because there's so much going on (and the loops make it easy to identify different aspects.)
- 4.23 / 5.00
- Adults Only
What I noticed about this was mainly that tentacles look better when their movement involves swirling or dragging, since they actually produce that kind of movement underwater. Pistoning motions are somewhat fantastic; snakes do them, but the movement is quite different from what you normally see in hentai.
I would opt for frame by frame but making it look good takes a lot of work.