As stated practise is key. I do have a few thoughts. Almost any ride I do over 50miles has over 3000' of climbing with grades as high as 21%. Seated is more efficient but standing to regain leg speed is also a good idea. I have no qualms with shifting while climbing and if you practise it should not be an issue, in fact you should learn how to! I can climb most of our hills in my 53-23 but on longer hills over 8% I will use my 39 and whatever. I usually start of seated on a long climb in a gear that I can feel the pull of the hill, once leg speed starts dropping I'll go one gear harder and stand till leg speed gets to what I need, then sit back down and go back one or two gears easier. It can be done just takes practise. I do find for bigger stronger rider I will try and attack almost every hill and use my momentum to get me over it, I'm not a huge watcher of cadance I use what gear I need.