Codespaces but open-source, client-only and unopinionated: Works with any IDE and lets you use any cloud, kubernetes or just localhost docker. - loft-sh/devpod
I have an idea though. Create a new tag. Call it release, include the Flatpaks in this tag, use the tag as the default in the installer, then post install you get rebased to latest without the Flatpaks built into the image. They should still be a part of the system