Sounds like Collabora will be the only option for now. I really look forward to that coming to disroot, even if the disadvantages as compared to Onlyoffice are quite disappointing. It will allow me to conduct my studies and reports almost entirly in-browser.
Until that gets resolved I can use collabora and when I really need Onlyoffice I can use that over webdav,
I hope Onlyoffice will add the key-mechanisms to its standard integration with nextcloud offering.
But thanks to both Onlyoffice and Disroot and Collabora for excellent services.