Well, its a fundamental necessity that if you want to share data, you must have some conduit for sharing that data. Without something like Nextcloud, what you're asking for pretty much can't be done. Yes you could use something besides Nextcloud for similar functionality (shared database), but trying to use google or some other 3rd party service for that means that you completely lose "privacy friendly", which is a basic requirement that you specified.
Besides Nextcloud+Cospend, you could use IHateMoney https://github.com/spiral-project/ihatemoney/ (which cospend is based on), but you still need to host it on some server, so you might as well put Nextcloud on that server and gain all the advantages of having a Nextcloud server.