I think this is still recoverable! But you need to first setup a bootable Big Sur OS installer drive. Using a second Mac reformat it to GUID and a journaled file system. Then copy to it the OS installer file, now follow this guide [https://support.apple.com/en-us/101578|Create a bootable installer for macOS]
Now with this drive plugged in an the original SSD installed you should be able to delete all of the partitions and reformat the drive as a single partition using Disk Utility, now run the OS installer. That should do it.