Welcome to TE, LunaB! Synapses are nice. Also, I am jealous of your scenery up there. Not so jealous of sharing the road with moose, though!

There are two things that I can think of that may be the problem. One is bike fit, if you haven't had one already. Even though the bike is the "right size", the fit may still be off. Set up an appointment if you can find a fitter.

The second is lack of core strength, which you picked up on. I don't have a list of exercises or anything because I find gym-type exercises incredibly boring, but yoga can help if that appeals. (I think there's "yoga for cyclists" or something similar out there, in fact!)

In the meantime, take breaks. Get off the bike and stretch a bit. It helps.