I don't know about reticulum but I wonder if you could (in theory, I'm sure it doesn't exist yet) have a semi-dumb node that can make basic routing decisions without implementing all the heavy stuff like crypto. because honestly there's plenty of compute available on a lot of these meshtastic devices for something like that, tracking node lists and at least a local route table, but not for running the whole stack

They're working on a C implementation to basically that, but it's not very closed to being finished yet. Reticulum is all Python now, which doesn't work great on microcontrollers. It's like this with a lot of mesh projects, where they have grand plans with awesome features, but stuff goes slowly thanks to a lack of developers.

running encrypted comms over anything ham would be illegal

Yeah, sadly. In America you could buy a license for a business band, those are right near the HAM bands and generally allow encryption, but the convenience drops off a cliff (at least several hundred dollars). Or you could move to a cool country without those restrictions.