Sorry to hear about your arm. You seem to be having a bit of a time of it.
Anyway, I have Talnotry campsite, on the A712 near the Goat Park, in my memory because it is one of those you could drive on to in the night after a long run up from England. It was quiet when I stayed, possibly because of minimal facilities, and very cheap, because the warden doesn't turn up till long after an active person would be wishing to do things, In other words, I stayed three nights and never paid, although I would have happily coughed up the backlog if the warden had ever arrived at a sensible time.
To the north west of Clatteringshaws, in the forestry and not far off forest roads are bothies, which are genuinely free and unlikely to be crammed with Southern Upland Way walkers in April. Hmmm, you might need one of those Nomads for that.