Moving boot disk - does not seem easy?

[james]

I had thought that this would be straightforward but it seems not.

I have a freebsd 12 system, UFS boots /ada0p2.  Mounts some ZFS 
partitions and I'm away.

I add a new PCIe card with a SATA SSD, and it grabs ada0.

I want to move my boot to the SSD, not least because the boot priority 
now favours it as ada0, and I had to manually boot ada1p2.

There's not much on ada1p2 now, but I want the new ada0p2 to be smaller, 
so dd is not attractive.

What's the easiest way to set ata0 to be much like ada0p2 was (given 
that I booted from ada1p2)?

Ideally I'd like boot and swap etc set up as well, which I kinda did 
already with sade.

I already had the issue with freebsd-install/MANIFTEST missing and did a 
basic install to ada0, but it seems a bit naff to unmount all my ZFS 
mountpoints just to tar across all the rest of it.

Any pointers?