Well, arse.
Fired it up, managed to get into Mod Manager and remove all the old ProMod mods that were well out of date and update any other mods I had which weren't compatible.
Started the game, was able to get into Garage, Truck, Trailer and Driver managers. Sold all the trailers and trucks that were showing as being at N/A garage and then sold all the garages that were showing as being at N/A cities. That's cleared all the ProMods stuff out and I now have over £20,000,000 but still have 40 fully populated garages.
So that's a result.
Now the problem is that the Logitech control software hasn't worked on the last two versions of macOS and there's no intention of them updating it. I have basic control but no clutch for the gearshift and only around 90° of turn goes from full lock to full lock. That makes the steering so twitchy that it's not really possible to drive.
My G29 and shifter is effectively useless.
I guess I should box it up, put it on the MM and see what the alternatives are. If any.