This does exist. There are Nextcloud Talk Chatrooms on https://cloud.nextcloud.com. You need somebody with the needed privileges to invite you to be able to join. I remeber that i was invited by Jan-Christoph Borchardt quite a while ago. It should not be hard to get an invite, you probably just need to ask.
The solution I found for me was https://www.gitpod.io. It lets you spin up dev environments in “the cloud” with full root access and Visual Studio Code in mere seconds. If you add a gitpod configuration file to your repository, you can make it automatically start a nextcloud server.
Gitpod is like Github Workspaces, but Open Source, exists longer than Workspaces, can be self hosted and the hosted version has a free version and cheaper prices.
If you want to go this route you can use my Gitpod config for nextcloud apps by copying my .gitpod.yml config file and the gitpod directory next to it and modifying it to fit your own app.