What are the advantages of epub over PDF? I've never heard of anyone asking for a conversion in that direction before.

Reflow.

Maybe convert it with Calibre?

I tried Calibre and it ended up with funky line spacing :(

Mine came out really well fwiw, reading it on Kindle right now

It's possible that https://pandoc.org/ could also convert a PDF to epub. YMMV, I haven't tried it.

Pandoc doesn't go from PDF to anything afaik