I've sent you a PM with some trip writeups, but yes, in short, European IFR flight is difficult without oxygen - not because you cannot generate routes at FL100 or so but because flying that low places you squarely into IMC which (most of the year) means likely icing conditions and almost inevitable turbulence of varying severity.
The name of the game is therefore a climb to VMC on top. In my book, this also means that the flight is scrapped if the cloud tops are too high - unless you have a solidly deiced aircraft.
OTOH there is a significant pilot population who "just fly" but I am not among them. The smarter ones are doing it in deiced planes, and preferably without passengers
Obviously, the well funded ones are doing it in pressurised planes, at FL200 plus.
Working out routings (there is an automated tool for it so you don't need the SRDs) works at FL070 but you get poor routes. FL100 is better. FL140 is better still and above that there is very little to gain except in the Alpine region where you need FL160. Then next big improvement comes at FL200+.