> Is there a Nextcloud / Owncloud alternative to Docs / Sheets for collaborative editing?

Yes, NextCloud and Owncloud both integrate with OnlyOffice Community Edition[0][1] which supports collaborative editing of text documents, spreadsheets, and presentations.

[0] https://www.onlyoffice.com/office-for-nextcloud.aspx

[1] https://www.onlyoffice.com/office-for-owncloud.aspx

Same as Google, afaict. It has a slightly better permissions model, imo (I can create a doc with a password and share the link freely, or I can give specific permissions per user, like Google). Same editing/commenting/viewing interface.

My collab needs are pretty light, I only work with a few family and friends, but I haven’t run into anything I wanted to do in Synology that I couldn’t.