You don’t have to redownload all your games. I just switched to the flatpak this past month, here’s what I did:
Leave steam installed natively, install the flatpak alongside it, using flatpak steam, open the root of the library folder (should be steamapps), then find the corresponding folder from the native install and recursively hardlink all the files from the native install’s steamapps folder to the flatpak’s steamapps folder. Quit steam and reopen and all your games should be detected.
It’ll need to “update” basically all of them for at least a second to verify the files and it might need to re-process Vulkan shaders, but that’s it.