Two Macintosh HD Data disks, one is taking up so much storage. Can I delete it?

You will need a Time Machine backup to do this, so if you don't, do something else. If you want Mojave, you need a backup you made while running Mojave. If you have made any files since you installed Catalina, copy them to an external drive because the backup won't have them. If you added photos to your Photos Library, export them to the external drive.

Oh ok by "erase the disk", which disk do I erase: the data or the data2, OR the Macintosh HD one?

By erase the disk, I mean erase the disk, not a volume, not some volumes. Erase the disk.

Boot into Internet Recovery (cmd-opt-R) and Erase the entire disk, then reinstall macOS. If you want Mojave, use cmd-opt-shift-R for getting into Internet Recovery.

How to reinstall macOS from macOS Recovery - Apple Support

And what do you mean by migrate from my backup during startup assistant?

After installing the OS, when it reboots, the Startup Assistant runs. It will ask if you want to copy your data from another Mac or from a backup. Use your backup. You do have a Time Machine Backup, correct?

How to move your content to a new Mac - Apple Support

If you have a backup of when you used Mojave, you can just restore from that after erasing the disk. Use the Restore from Time Machine backup after quitting Disk Utility. You won't need to use the Migration Assistant.