For WOT you probably don’t have to wait until that high a temp, but for auto-x or tracking I’d wait as long as is possible since you’ll be asking much more of the engine dynamically. The thermo on that STETT oil cooler kicks in at whatever temp it may, but it’s not like it literally freezes the temp in place, it will still rise when you get on boost. It just does a better job of anticipating the temps rising, and pinning them down when they get to 230 or whatever it may be depending on other factors. Personally I try to stay off serious boost/stay under 3k revs until the oil temps get into the 200 range, though once the needle gets off the 160 peg I don’t worry too much if it needs a bit of gas. Maybe I’m just a bit anal like you, the thought of “hurting” my car scares me. Your approach is certainly not going to cause trouble, so if it's what makes you comfortable, it's all good.
Oh and I don't know any N54s that sit at 210 when everything is hot and the car is being driven with decent load, unless it's extremely cold outside. "normal" temps i see with stock oil cooling are more like 220-250 depending on the outside temps and load. Then again MD is probably a good deal hotter than the UK for most of the year.
E88 N54 Alpinweiss/Coral Red/Motiv HTA 3586r Tial .82AR/Other stuff...652WHP
F30 N55 XDrive EBII