As I already model in P4/S4 standards, I'll be using P4-profile wheels, set to 19.9mm back-to-back. I know I won't be able to run my eventual "S"-class 4-4-0 around tight curves, but that's not the point of P4 standards anyway.
With the MJT torsion bar bogies widened, that's a simple way to get a working suspension. I'm not sure if Bill Bedford's sprung bogies are available for 21mm gauge, but would be worth finding out. Alternatively, it might be possible to widen his standard gauge ones. They are now under the ownership of Eileen's Emporium.