Thank you @meaton. It appears that things didn't improve in Monterey developer beta 3 (21A5284e): installing and enabling a network extension appears to drop all internet connectivity on my Mac. Private Relay was disabled the whole time. I've reported this issue as well with a sample project: FB9381341. Let me know if I can provide any additional info.

This is batsh*t insane, Apple has been sitting on our proceeds for 7 months, with 0 communication. Last interaction: 15 days since I wrote to apple finance via their support form, sent another follow up, 0 replies. Considering legal options.