@Erunn You could use ONLYOFFICE with #Nextcloud. You could host it yourself if you wanted, or use one that's already setup, like #Murena.

Cryptpad has worked good for me so far.

Take a look at Cryptpad Forms or FramaForms.

Someone already mentioned ONLYOFFICE, another nice Nextcloud compatible office suite is Collabora Online. In my experience it has the best support with open document formats (.odt, .ods, etc.) since there's LibreOffice at its base.
They also have a (not top notch but still working) open source client for iOS and Android. For the latter they even created a specific F-Droid repo.

What kind of legwork are you willing to do? Do you have access to an old computer capable of being used as a server?

I have not used this and can't vouch for it, but some digging led me to ethercalc. On the surface it looks looks legitimate enough to explore further. As I always would when installing anything or using an online service with sensitive data in play, I highly encourage doing your own research to be sure that you feel comfortable that it's legitimate and safe, but I didn't notice any glaring red flags.

Cryptpad