Pull up on the pedals.
Assuming weight is not an issue and other aspects of cycling are good including diet, pedaling in circles, conditioning etc…
The one thing I’d add for cycling in honest mountain climbing is train PULLING UP ON THE PEDALS. That often gets neglected otherwise because it’s not needed. One reason why it’s difficult is because with out being in tough steep situations, it’s easy to shift down or slow down, because the simulation is so hard. You could just shift up a number of gears and force yourself to train at times like that, and if you’re in a mountain deficient zone, that’s your only recourse.
The concept is riding on 7-9% sustained ramps with a 42x25 is very doable. In good shape a cyclist can spin, pedal in circles and / or power a sustained 7-9% climb and survive with out much pulling up on the pedals within reason, with associated welcome suffering & it’s wonderful buzz for 20+ miles. Seek out the next dimensions and reality puts cyclists in a different position to do everything possible in our galaxy to fight like salmon going up a waterfall. That requires among other things pulling up on the pedals like crazy in a sustained calculated way, which is also dictated by the length of the climb to reach success. Pulling up IS the difference in going forward or not.
Pulling up on the pedals on very steep climbs will help pick up speed and help with success on average 7-9% ramps.
The absolute requirement of pulling up on the pedals on 14+% climbs puts cyclists in a position to not only pull up (along with pedaling circles) but builds muscle memory in advance of doing it when needed on say 20 mile climbs. On huge 14+% climbs there’s no getting out of it, no getting tired and slowing down, dropping gears or changing commitments. You dig in, that’s the only course.
Train for that because it’s coming whether you trained for it or not. Do it beforehand and the soreness those muscles experience for a few days will occur now rather than in Europe when it may shut you down for a couple days.
I get a lot of 7-9% ramps on 4-10 mile mountain pass climbs, where nearly every ride is over 9,000’ above sea level and seek out 11-15% grades for training for those 9%ers. Many climbs are 15-20+ miles. The very steep ramps really help for those ordinary climbs. You could also get a granny to help but…
Don't forget to talk to your legs: SHUT UP LEGS, helps Jens... And if it's good enough for Jens...
Which brings up elevation. Europe may not be as high as Colorado but if you’re in a lower elevation it will be another factor you must consider.
Ultimately, no matter what, other cyclists go ride the mountains in Europe and you can do it too.