Is putting Server 2019 (full os with hyper-v) and 2 2019 Guests on a single two-disk Raid 1 array reliable enough?

Our main goal is reliability/redundancy, so would 2x 1TB SSD drives in Raid 1 be reliable/performant enough in this case?

Reliable enough for what? Define that first.

RAID1 would protect you against a single disk failure but doesn't protect you against any other type of host failure.

I'm thinking partition the disk so the server 2016/2019HV (~128GB) is on one, and the two vm guests on another (~812GB).

Don't. Just don't. This doesn't provide any benefit and only serves to make things more complex than they need to be. If you want to separate your Hyper-V install from your VM storage then use separate storage altogether.