I think you do not understand OnlyOffice and Collabora Online. First you can use both read/write in the web browser (Webapp). Also you must not install OnlyOffice and/or Collabora Online on the client. It is a server side installed office. There are, however, limitations with mobile clients or also with the use of a free version.
You can use test accounts:
OnlyOffice: Demo - nextfiles.de
(includes OnlyOffice, but it is Nextcloud 21 and maybe OnlyOffice is outdated)
Collabora Online https://nextcloud.com/instant-trial/
You must register with a valid email for a 60 minutes account
Collabora Online is integrated with the name Nextcloud Office
Also you get at some free hosters also (mostly) OnlyOffice. OnlyOffice has got the advantage (for hoster) that the most cpu is used on the client (Javascript) and not on the server.
But there is a difference if you install OnlyOffice via the Nextcloud App or via Docker. How did you install it?
But i don’t understand all these problems with the licenses although it is supposed to be “free” software at least in parts. Maybe someone has a source where this is explained properly.
And I am not able to open a document in “writing”-Mode. Only Viewer. I installed it 2 weeks ago, so it’s the latest version. I don’t understand.
I installed a viewer. Nothing more Or i just don’t get it.
For me OnlyOffice is much better than Collabora as app, better performance for normal use, better specs, better UI, etc. but the community version has some limits like concurrent users or mobile editors and for some users could be a problem.
Both are difficult to get ready to run and 9 of 10 times you will have problems in your first try and the Community Document Server should be removed from the store because has very BIG bugs, when it was released was a huge step for NC and was easy to setup and install but seems an abandoned project. If you want to give a try to OnlyOffice you should install the server in a docker or a separated machine (and cross your fingers).