Realism vs. Pleasurable has always been intensely discussed since MSFS was initially released.
Many sim-pilots use 3rd party ATC products to enhance realism. All of them have their strengths and weaknesses that also have been debated many times here. The “search” function should help you find posts/topics of interest.
One of the strongest features of 3rd party ATC is the ability to actually talk with someone making it as realistic as possible.
The built-in ATC is text based that is displayed and spoken which is clunky at times and not highly realistic. However, it is very structured and uses FAA/ICAO terminology. It is a good way to learn how to communicate with ATC using specific ATC terms and instructions.
How ATC communicates altitudes is another huge topic. In the past ATC used to give crazy altitude instructions depending on who created the flight plan. It works a lot better today using SID, STAR, and approaches altitudes correctly. Probably the weirdest instructions I hear these days is when flying a missed approach, there is a hand-off to a Departure controller. They issue an climb instruction to 10,000 ft. Ten seconds later, they see that you are flying a missed approach so they will then issue instructions to descend to 4,000 ft and then a vector to restart the approach. One other issue is the “Cruising Altitude” in the Navlog/Flight Plan. There is no error checking by ATC. If the cruising altitude is 32,000 ft and you are flying a C172, there is no way for the aircraft to go that high. However, ATC continues to issue climb instructions because they don’t know you are flying a C172.
There are jets and airliners that do not import MSFS flight plans directly into the FMS. Vendors usually have their own process to import various flight plan formats. Also, many people will take the time to manually type in their flight plan. Two-way communication between the FMS and ATC is not very well implemented yet.
Finally, you asked about two high altitude airports. If you fly VFR direct between the two, you are responsible for flying around obstacles, mountains, clouds, and other aircraft. ATC will not issue low altitude alerts. If you use standard IFR charts and stay on a published airway, ATC normally issues correct altitude and speed restrictions. If you fly direct from one waypoint to another not using published charts, you no longer have any altitude protection from ATC. However on IFR charts there are minimum obstacle clearance altitudes published for each sector. ATC doesn’t use these altitudes but you should when planning your flight.
I have flown one specific published STAR where ATC issues vectors to the assigned approach. The destination airport is in a valley with high mountains on each side. For some reason ATC gives me vectors that are too low and crashes me into a mountain. One reason for the problem vectors is the speed of the aircraft. Airliners make wide turns avoiding the mountain. Small aircraft fly slow enough to make small turns avoiding the mountain. This is the only instance I know where ATC flies you into a mountain USING VECTORS. There may be others that are repeatable.
Finally, you mentioned going around the world. MSFS has only one ATC but the world often has a separate ATC for each country or region. Every time I cross an ocean I seem to always be in contact with ATC. IRL VHF signals cannot span oceans so aircraft use HF or satellite communication neither of which are in MSFS.