The biggest chunk of work I see with the projects is availability of packages in Debian. It is also the area where we can collaborate easily without any other complication over technical decisions. I guess if we work together to make happen packages like Mastodon/Pleroma/NextCloud in Debian. Yunohost can have nice upgrades paths due to Debian packages and FreedomBox can have the apps available in first place. This will also benefit the larger Debian ecosystem considerably.
We can also collaborate on the setup scripts but that is much smaller effort compared to making the app available in the first place and then keep it sane during upgrades.