Man you gotta go with the brass! I was in your exact spot before a huge trip to Berlin actually... spent weeks comparing the weight and torque specs on these things lol. Honestly, the Match Technical Thumbs Up EP-MX Black is the absolute king of ergonomics because of that specific curve they use. It puts your thumb at such a natural angle compared to others.

• Match Technical: Solid brass construction, amazing hand-feel, basically zero wiggle. The 22g weight adds a nice balance to the M10 body and the silicone insert protects the hot shoe perfectly.
• Leica Official: Leica Thumb Support M10 24014 looks clean but the price is just wild. Its also brass, but the thumb rest part is a bit flatter than I like for 8+ hour shooting days.
• Haoge: The Haoge THB-M10B Thumb Rest is fine for the price, but its aluminum. It feels kinda cheap and doesnt have that dense, high-quality feel that brass gives you. I used the Match Tech for ten hours straight walking around Mitte and my hand felt fantastic! The way the brass wears down over time to show the gold underneath is just beautiful too. Since youre dropping M10 money, dont skimp on the one thing that keeps it from hitting the pavement! Grab the Match Technical and dont look back, seriously its life changing for street photography... my grip felt like it was basically glued to the camera.

Stumbled upon this discussion today. Building on the earlier suggestion, I'd urge you to focus on the mechanical fit rather than just the ergonomics. In my experience over the years, the hot shoe is way more fragile than people think. The biggest risk is leverage... if a grip doesn't sit 100% flush, it acts like a crowbar and can pull the hot shoe assembly right off the internal frame. I've seen M bodies ruined this way. Honestly, if whatever you buy has any wiggle or requires force to slide in, stop using it immediately. You don't want a loose shoe halfway through your Berlin trip. I've tried many over the decades and usually, the ones with poor tolerances are the ones that cause the most damage. Just be careful not to trade your cameras health for a bit of thumb comfort... it's a bad deal.