5G Standalone networks don’t have 4G to fall back to. 5G Non-standalone networks are essentially 4G networks with a 5G RAN, so SUCI remains optional and most core vendors don’t support it.
That's not what 5G standalone means, as far as I understand.
The network I'm using supports 5G SA in some cells, but my phone definitely still falls back to both 4G and 5G non-SA in some areas where it's not yet available.
And even if 5G SA were available everywhere, there's the concern of roaming.
Correct, your phone needs to actually re-connect between the two networks. It's a whole new session and you can't handover between 5G SA and 5G NSA/LTE networks. There are some configurations that make this not much of an issue, but technically they are totally different networks.
