WiFi extremely slow after upgrading to Big Sur

To start with, I know this is a very common issue and I've read probably 100 posts in this forum looking for solutions, but so far nothing I've tried has worked.

I'm on a 2019 13" MacBook Pro and over the weekend upgraded to Big Sur (11.6.1) from Catalina. I knew that degraded wifi speed was a potential issue, so for the weeks leading up to the upgrade I checked my speed with Speedtest.net (and their app) and was routinely seeing speeds of around 500-700mbps up and down while connected to wifi.

Since upgrading, though, I'm only seeing speeds of 40-50mbps up and down while on wifi. If I connect the computer directly to ethernet and turn off wifi via the toggle, I get speeds of 900mbps up/down.

I've isolated this as an issue with just this machine: I have a 2019 15" MBP that I use for work and it's getting 500-700mbps on WiFi—but it's still on Catalina. I have two other, slightly older Macs in the house that get 500-700mbps on wifi, too, but they are on Mojave. ****, my iPhone 11 Pro pulls faster numbers than my poor MacBook Pro now. I've tested this machine on a friend's wifi network (that has gigabit internet, too) and it's slow on it as well.

Based on advice in this forum and scouring the internet, here are all of the things I've tried to no avail:

• Reset SMC and NVRAM
• Ran wireless diagnostics (system was found to be running fine)
• Created a new wifi configuration by turning off wifi, deleting the following files and restarting the computer:
  • “NetworkInterfaces.plist” “com.apple.wifi.message-tracer.plist “com.apple.airport.preferences.plist” “preferences.plist”
• Created a new network location using custom settings
• Disconnected from my wifi network, deleted it from the list of preferred networks in System Preferences > Network, then reconnected.
• Booted into safe mode and tested speed once in safe mode—it was still slow, same speeds
• Created another admin account on the computer and logged into it—speeds were still slow
• I read this post that indicated a conflict between wifi and bluetooth could be the culprit, so I followed the steps outlined to create a new com.apple.bluetooth.plist file but it didn't help
• Restarted my computer after doing each of these troubleshooting steps

I even restarted my router (Orbi RBR50 mesh system), though I know it's not the issue since other devices are seeing much higher speeds and this computer was, too, before the Big Sur upgrade. I did all of my testing either sitting directly in front of my router or sitting about six feet away from it. There is no interference that didn't exist before the upgrade, but there really isn't any interference anyway because daily I sit about six feet from my router.

I do have AppleCare on this machine, but I am reluctant to mail it off to Apple if this is something that can be solved otherwise. I would go ahead and upgrade to Monterey but I see plenty of forum posts about the same issue after upgrading to it, too. Is there anything else I can try that I've missed?