I had the exact same issue with cheap third-party grips on my 80D before upgrading to the 90D. The voltage drop and connection drops during high-speed continuous shooting happen because those cheap knockoffs use subpar contact pins that cant handle the amp draw or the flexing of a heavy lens. Honestly, you should just buy the official Canon BG-E14 Battery Grip. I bought mine a year ago and have been incredibly satisfied with it. The grip works well in wet conditions because it actually has the same weather-sealing gaskets as the 90D body, and the connection is rock solid. No complaints here, even when tracking fast-moving birds at 10 fps. For the strap, carrying a heavy lens on your neck is a recipe for pain. I switched to the Peak Design Slide Camera Strap SL-BK-3 and it completely saved my shoulders. It distributes the weight across your collarbone and back when worn as a sling. The quick-connection anchors are rated for 200 pounds, so no worries about gear dropping. I mount one anchor to the left eyelet and the other to the tripod plate on the bottom of the Canon grip, and it hangs perfectly. Let me know if you need any help setting up the mount plates, I can definitely assist you with that.

Building on the earlier suggestion, I also learned the hard way about cheap gear when I first started out. Tbh, I almost dropped my camera because of a loose plastic grip. Swapping things out for a mid-tier setup made a huge difference and I am super satisfied with how it performs now.

• Vello BG-C10 Battery Grip for Canon 70D 80D 90D
• This runs around 80 dollars and the build quality is night and day compared to the knockoffs. The connection pins actually fit snug so it never shuts off mid-burst.
• Op/Tech USA Utility Strap Sling Quick Adjust
• The thick neoprene padding on this is a lifesaver. It stretches slightly to absorb the bounce when you walk, making a heavy lens feel way lighter. Going this route easily keeps you under that 200 dollar limit while still getting reliable gear.

Re: "I had the exact same issue with cheap..." - yeah, those generic grips are basically paperweights when you hang a heavy 100-400mm off the front. I actually disagree a bit on the Peak Design suggestion from earlier tho. While they're popular, for heavy birding glass in the woods, a cross-body sling is much better for your neck long-term. The torque from that big lens puts huge stress on the grip connection. If you're trying to save a few bucks, try these:

• Vello BG-C10 Battery Grip for Canon 70D/80D/90D - it has much better contact points than the generic ones and actually handles some moisture.
• BlackRapid Sport X Camera Strap - this uses an underarm tether so the shoulder pad doesnt slide around while you're hiking through the brush. Since you're dealing with PNW drizzle, just be careful. The Vello is decent for the price, but for total peace of mind with the 90D, the Canon BG-E14 Battery Grip is the only one I'd trust to keep the weather sealing intact. Good luck with the hawks.