Found this post after months of struggling and following other suggestions online. Putting in a DNS address made my WiFi stable enough to download the Big Sur 11.3 update. Installed it and rebooted, removed the manual DNS entry and I've been online for 2+ hours now. Huge improvement! I think the bug got squashed for me.

I had created feedback and submitted my bug report multiple times, and opened a support ticket with Apple and none of the suggestions fixed it. So I just installed Big Sur 11.3 today after struggling with this on my MacBook Pro 2018 for months. Putting in a manual DNS entry was the fix that allowed me to download the 11.3 update consistently (this was from another forum post). Once that was done, I removed the manual DNS entry and been online for 2+ hours now! 11.3 seems to have fixed it.