I drag my back brake around switchbacks. It really seems to help keep things under control. I think I learned it from a GMBN video.
Also on super-steeps, I lower my dropper post a few inches to shift my center of gravity away from the back wheel.
If I were buying a new bike, I'd only upgrade as required to ensure it had SPDs, dropper post, 4-piston brakes.
Aside from that, I generally don't upgrade anything for a good six months at least. I think a lot of people just spend money on needless upgrades when the stock parts are perfectly fine.
Maybe an old-school thumb shifter?
https://www.microshift.com/models/sl-m12-r/
Despite the name, you can pull with a finger to go into a harder gear and push with your palm to go into a easier gear.
I'd take the e140 hands-down. It's better in almost every regard (assuming the comparison on the Merida website is the same as what you're looking at).
I run a sort of hip pack at the moment but due to being a fat bastard, it hangs quite low on my hips. I have to do the belt up quite tight which gets uncomfortable.
I’m quite tempted by the Road Runner Burrito bag because it would fit in the space under the stem and bars.
Well, I’ve tried all the tricks and I’m still getting regular creaking. One thing I’ve noticed is it appears to creaks after I stop pedaling, but stops when the overrun does. This leads me to think maybe it’s a problem inside the motor.
I personally went Shimano because they use mineral oil so bleeding them myself is a bit easier/safer.
On the same channel as the install guide, he does a deep-dive video into those exact M6120 brakes.
A bigger rotor will help a bit but you really want four piston brakes to get the good braking. If you want good value, get the Deore MT6120 brakes. I'm significantly heavier than you and they stop me really well...