They can't reach 20000 ft or 6000 metres.
Some can fly at 6000 metres, but other models have service ceilings of 4000 metres.
And if you are thinking of flying from low levels to those altitudes, that's just asking for trouble.
I am from Germany and have a Mini 4 Pro. All sub-250 gram drones (also C0 classified ones) don't require remote id here and the actual firmware for my Mini 4 Pro doesn't support it yet.
However, if I would like flying with extended batteries and...
Don't think so, it is C0 class drone like (I don't have mine here, so cannot check if it got C0 or is a legacy one) and that class is not required to broadcast any ID in Europe. A C1 class drone has the possibility to enable it. One thing I have...
SDK / Third party apps aspect
At least each recent generation of drones got now a "pro" version of the RC. The remaining big question mark is what drones / RC pro 2 will be included in next SDK version so it can be used by third party app...