I've been trying to accomplish the exact same thing. In the same vein I've also been trying to setup a tailscale exit node with mitmproxy so that i can inspect mobile app traffic without having to fiddle with proxy configs on my phone each time. In relation to that i found this - https://www.aapelivuorinen.com/blog/2022/09/12/transparent-mitmproxy-tailscale-vm/
Let me know how you get on as I'm super keen on having both a VPN and mitmproxy setup as exit nodes.
I did thanks, will be trying it out later today. Cheers.