GPS tech (Garmin, Hammerhead, Wahoo, RWGPS, etc)

Nope, mine's a trekking GPS more for hiking and stuff than cycling or driving. It's a Garmin eTrex Vista HCx.

I'm trying to work out how to get the free OSM maps working with it.

Lot of water in that map. Were you heading for it?
I have a cough copy of OS memory map, with a full set of maps, not sure if it will work with your Garmin, but you're welcome to give it a try