The most success I ever had with clematis was due to completely ignoring it, expecting it to just die like previous plants so not investing effort.
It's doing fine now.
They are rampant but don't actually need pruning unless you want to keep them in check. I have had several over the years that I never touched and they filled walls up to roof height.
Clematis montana will quickly grow into a twisted gnarly mess if they are left unpruned. Do take a bit of time to cut them back as soon as the flowers have died off. Group 1