It's starting to get tough. I see that across most of the communities except for a select few. A few posters keeping communities alive with mostly just upvotes/downvotes as engagement. It's a difficult role to have, especially long-term.

The reality is probably that the group that is here right now is probably the group for a while. On one hand, it's cool that we don't get instantly downvoted to -1 within seconds, it also means that your posts don't get upvoted to the thousands or have 100 comments. I personally feel that there is a point of critical mass where this instance just takes off, due to some random event but sometimes I wonder - do I really want that? All those people bring drama, reposts, bots, DMCA, and low-budget content. It does bring OC, creativity, and variety, which is really missing at times.

Anyway, to your question:

I tried a banner contest once, it just sat there with mostly no response - https://lemmynsfw.com/post/1290450

I did a poll once, I got some good feedback using pictures as options - https://lemmynsfw.com/post/887466

I think starting with low effort engagement is a good segue to better engagement. Ask questions, make a poll, make a tournament of pictures, etc. Go back to how large forums used to do it. I also believe that as folks get used to each other it will be come more engaging. It takes time. Also, for your video question, .webp is king at the moment. See here - https://lemmynsfw.com/post/884574