Someone on another forum found the solution to my problem, so I'm posting here in case it might help someone:
''Boot to a High Sierra installer, and the internal drive will be now visible, and you can erase, or do anything else that you need to do with the internal drive. If you still need to erase the internal drive (and you do need to erase that drive to do what you want with a Sierra system!), then you have to erase the drive with a system newer than Sierra. High Sierra will be successful. Once the drive is erased (and you have reverted back to a Mac OS Extended format), then you will be able to install Sierra -- followed up with reinstalling Monterey. You will want to decide how much drive space you want for each system. I think you will also realize that you will perhaps struggle when choosing to boot from one system, then the other on the same drive. There would be workarounds to get that to work, too. If you want to have both systems, because of your legacy software, then that will be something that you will have to deal with.''