Agnosticism over protocols (maybe ActivityPub won’t be the only protocol we want to support in the future)

If that's simple to do, it's worth waiting for, but if it's trickier then I've learnt the importance of remembering YAGNI and KISS (and that database migration scripts aren't that hard). Of course, third party database queries aren't as simple to update but perhaps your API will be all implementers need.

If we can get under 16 columns, we can disable support for 32 columns in Diesel and save build time

This does seem worthwhile, just to speed up development cycles.