A couple of noteworthy quotes for you and anyone else interested to ponder from irl airline pilots if you are going to try to keep the realism in the sim as much as possible !!!
Most passenger airliners do NOT fly over 40k. The 747 can get up to 41 or 42 for regular flight sometimes. The 737 will generally not go above 39. Most airliners CAN fly slightly above 40, but not above 45. Most will not. There is a higher risk of engine starvation and flame out, along with a drop in power, efficiency and control response.
While quite a few business jets are certified to 450 or 510, staying up there for an extended period of time is a pain, mainly because one of the pilots has to be on oxygen at all times at 450 and above and that gets very tiring.
The 747, at least some of the older ones, were/are certified to 43.100ft, and that’s as high as a commercial airliner is ever going to go these days - but it will be an incredible rare occasion, if ever.
You will rarely, if ever, see a US airliner going above FL410 because, if they do, one of the pilots must be on oxygen for the entire time above FL410
Class A airspace tops out at FL600 but Class E airspace, also controlled airspace, continues up indefinitely. FWIW, the max certificated altitude for the B767 is 43,100’