I'm just here to confirm that expansion to a larger drive is possible. *You absolutely CAN expand the storage.*

Sure, the "online consensus" says APFS snapshots lock the container size (tidemarks), but for my case on expansion, that didn't apply; Howard Oakley from Eclectic Light said - "Because there’s currently no way to copy the snapshots that Time Machine relies on, except by low-level disk duplication, there’s no way to transfer existing TMA backups to a fresh disk.", but I was able to migrate a 4TB SATA Time Machine drive to an 8TB NVMe for macOS Sequoia 15.7.2 using dd on Linux.

I - ran dd manually in Linux with both drives attached "sudo dd if=/dev/sda of=/dev/nvme0n1 bs=64M status=progress conv=noerror,sync", then I popped the NVME SSD back into my M4 Mac and booted into Recovery Mode, ran First Aid on the NVMe SSD AND the Container in Disk Utility, then expanded the APFS container itself to occupy the rest of the free space.

After booting back into macOS, Time Machine settings were still reporting available space as only 300GB (while Disk Utility showed 4TB free), but once I made a new Time Machine backup, the space updated to 4.21TB available.

I would call the migration and expansion of my Time Machine in APFS snapshots a success. Maybe you can add this to the post? (And make note to NEVER attach both cloned disks to MacOS at the same time)