I would go for hedgedoc if the team can switch to markdown, otherwise Cryptpad to stay in an office-like suite.
If you already have nextcloud then that + collabora should be good enough. Otherwise installing the whole nextcloud suite just for collaborative editing seems like a lot imho.
Dollphone is definitely not flat or at least not the latest minimal/abstract style
https://f-droid.org/packages/me.x2gd4.dollphone/