So, in case this helps anyone else, I was able to finally get it working with Mullvad exit nodes. I had to follow the instructions below from another post.
I found the answer to my problem of mullvad exits not working on my Beryl: > Configuring the Exit Node Using the LuCI Admin Panel
To successfully configure the exit node, follow these additional steps in the LuCI admin panel:
Access the Advanced Settings:
• Navigate to System > Advanced in the LuCI admin panel.
• Select Network > Firewall.
Edit the Firewall Zone:
• You will see three zones listed by default:
lan > wan
wan > REJECT
guest > wan
• Locate the second zone (wan > REJECT) and click Edit.
Modify the Advanced Settings:
• Go to the Advanced Settings tab.
• In the “Covered devices” dropdown, select tailscale0.
Save and Apply Changes:
• Click Save, then Save & Apply to finalize the configuration. (Unable to use tailscale exit nodes on Beryl AX - #18 by rairoumaru)
I'm not sure why this is so hidden. But it my case it seems to be unique to Mullvad exit nodes as normal exit nodes seemed to work fine.