Yeah, I think SilverBullet can't be of much help here. It's not built for multi-player collaboration. You could theoretically do that a little bit, but I wouldn't recommend it. What I sometimes do in similar situations is write a first draft in SivlerBullet, then through its export command (Cmd-e or Ctrl-e) export it as rich text and paste that into Google Docs. That works. It's a one-way operation though.
This is a super common trap. I think a lot of us get side tracked into working on the system than actually using it. In this context, sadly, SilverBullet gives you virtually unlimited power. It's up to you to restrain yourself
Yeah, I tried SB on Pikapods and indeed you can enable SFTP access, which seems to work fine. I don't know Devonthink and if you see that as a replacement for SB or companion.
If you just need the certainty that you can get your files out of Pikapods, you should be fine with their SFTP access (you can just download everything whenever you want). I think they also have S3 backups. What you can also do is backup/sync your files to e.g. Github: Backup your space to Github
If you host SB yourself on your own server, you have a bit more flexibility of course, but it all depends on what you need.
I think this is all doable, it's more or less what I also how I use it.