Hi,
I would like to thank @MartinL for his clues on how to make a bootloader for a different SAMD chip. This thread was really helpful.
I now have a working SAMD51G18A uf2 bootloader based on the Adafruit uf2 bootloader for the ATSAMD51G19A for the ItsyBitsy M4. This means it's crystalless. I offered it to be hosted on the Adafruit repo and they said if others are interested in using this bootloader, they'll host it. If not, I can just keep it in my private fork. So I'm just asking, is anyone else interested in this?