You can unlock the 20 concurrent connections with LibreOffice Online (not collabora) for free by compiling it yourself.
It’s a pain in the *** but i made it once. And a pain to update it too.
You can do the same with OnlyOffice but the documentation isn’t updated so for the moment you can’t compile it by following the documentation…
Again if you don’t want or can’t afford paying the license, you can unlock the limitation by compiling it.
OFF TOPIC : it could by great if someone can make a docker image of unlimited LibreOffice Online (LooL) or Onlyoffice Document Server.