As it looks like the devs won’t bring back the feature, I tried LibreOffice Online (Collabora Online Developer Edition). There’s a lot of material online and in this thread stating that it is inferior compared to OnlyOffice in a lot of ways.
After trying it out today, I have to say that I’m impressed.
The mobile browser editing is far better in my opinion than OnlyOffice. It may be generally a bit laggy especially during scrolling, but the input mechanics on a touch screen seem more reliable to me.
While it may not run that well on a RaspberryPi (if it runs at all on ARM?), my i3 CPU from 2017 with 16GB is handling a spreadsheet file without a hitch. Memory consumption is about 1% when opening a text-only spreadsheet - I expected it to be much worse.
Installation was also very easy via docker. I ran into a problem at first because I’m not able to use subdomains, but I had some help from people on this forum. All in all, it boils down to running one docker command, setting up the webserver config and putting Collabora’s outside facing service URL into Nextcloud’s collabora app setting.
Since LibreOffice’s MS compatibility capabilities have been enough for my needs in the past, Collabora seems to be an even better replacement in my use case, rightn now.
I’m thankful to the OnlyOffice devs for pushing me to try it out 