Some positive news... I managed to ride it for 10 minutes or so and not die. Got dark and cold so came home sooner than planned, got the weird looks from people.
Swapped the solid axle on the front wheel for a qr, put the (cheap/new) slick bmx tyre replacing the knobbly cracked one, fitted the steering damper spring, and tightened the bungee elastic underneath.
Whilst forcing myself to relax, I could feel some wobble but it wasn’t deathly. Maybe with more spring tension that would be further reduced, but it was okay.
On slow sharp turns the steering was trying to snap back straight so there was a wobble I was fighting with, but otherwise the wobble only really came out going over bumps. I noticed it a bit when applying some super killer legs power uphill, but only on the first or second pedal stroke.
Turning takes some getting used to, it isn’t that slow, but probably me afraid to steer too deep and risk the oversteer and slam.
Managed to go faster and it felt stable, just the wobble with all the awful uneven paths around.
Will try my hand riding it a few miles tomorrow, to see if I can make it even half the distance I plan to use it over. Never realised how winding and hilly my routes are.