I've tested a lot of gear over the years. Most of it was unnecessary. Here's what actually matters after hundreds of days in the Himalayas.
Shoes: One pair of broken-in trekking boots. Not new. Not fashionable. Broken in over at least 50km before the trip. This is non-negotiable.
Base layer: Merino wool. Not synthetic. Merino doesn't smell after 3 days. Synthetic does. When you're sharing a tent or a homestay room, this matters.
Rain jacket: A proper one. Not a poncho, not a windbreaker. A waterproof, breathable shell that you've tested in actual rain. In the Himalayas during monsoon, this is your most important piece of gear.
Headlamp: USB-rechargeable. You'll use it every single night in a homestay or campsite. Bring extra batteries anyway.
The gear that doesn't matter: Fancy trekking poles (a stick works), hiking pants with 17 pockets, solar panels (your phone should be off), and anything that weighs more than it should.
The best gear is the gear you forget you're wearing.