To alleviate some of your fear: I have seen zero diseases with apps. That’s from 23.10 to 24.04 then 24.10 and now 25.04. They work great. All my jails (all two of them ;)) were easy to move over as apps, as their data was already in a dedicated dataset.
That you definitely want, a dataset for the app data, independent of the iocage dataset.
The one thing I have seen in the forums are people struggling with 40+ apps on HDD and a hard-coded 120s timeout value. Fixed in 25.04, and maybe also those users may consider a) apps on SSD pool and b) why on earth 40+ apps. But if one must, then on SSD seems saner.
This looks like a good guide for nextcloud plus collabora with ngnix as the proxy: A full guide for installing Nextcloud on TrueNAS Scale with Collabora integration and outside access through Nginx Proxy Manager
I don’t think you need Incus for this, Nextcloud works just fine as an app. You can of course wait for 25.04 in April and make it an Incus container, more like a jail - but for this specific use case I’m not convinced it gains you anything significant.
You can set this up as completely separate hardware, but I don’t think you have to.
Make sure datasets holding nextcloud data outside iocage exist and have both snapshots and backups, make a backup of the config including key for encrypted parts of it.
Try the nextcloud setup fresh on a CE or SCALE VM, just to get a feel and take notes so you’re not learning during maintenance.
Remove the boot media, put in fresh boot media, install and restore config, run through your nextcloud setup as documented earlier, connecting your existing datasets for the data.
Do NOT update pools until a month or two of solid running.
All in all half a day to a day of switchover effort, not counting the time to dry run the setup - “just” announce there’ll be a little downtime.
Push comes to shove, put the old boot media back in, you’ll be right back where you were. Go back to drawing board to figure out what step might have been missed in migration. That’s worst case and I trust you’ll have sufficiently good notes from the dry run that you won’t need to.
If you’re really keen on not using apps and prefer maintaining nextcloud yourself inside something jail like (though see earlier - maybe more maintenance effort than desired without much or any gain), then you can reasonably try that idea on a CE VM with Incus container, then implement in April or May when CE is GA. No real rush after all, your CORE jail still works.
My experience has been that SCALE and now CE feels easier with apps than CORE ever did. Pre packaged apps actually work, I don’t need to manually create jails, I don’t need to hop into jails for updates and maintenance (or create cron jobs for that). After deployment, every so often I can click “update all” in the UI, and it does, and everything “just works”.
Apps survive across updates, even with the switch from k3s to Docker. I don’t need to manually handle OS updates for/in the jails.
I was wary of the switch from CORE to SCALE, which is why I waited for 23.10 before going for it. Turns out it was easy, and my fears were unfounded.
I did hold on to the iocage dataset for a really long time - finally deleted it just prior to moving to 25.04. After a year and a half, I’m now confident I won’t ever need to move back to CORE for any reason.