Planka is a good option, altough its more like an alternative to Trello. You should check it out and see if it works for your use case

For smaller folders I like using syncthing, that way it’s like having multiple updated backups

You can check their capacities and speeds with software like cpu-z.

This is the correct way to do it, use it, see if it works for you and try to understand what happened. It’s not that different from using examples or stack overflow. With time you get better, but you need to have that last critical thinking step. Otherwise you will never learn and will just copy paste hoping it works

I’m looking for the same. I’ve used moon reader on android and it can sync progress between devices. Also it supports WebDAV, so calibre may work but I haven’t tried it