Tailscale just partnered with Mullvad so this works out of the box for that setup: https://tailscale.com/blog/mullvad-integration/
Ah I literally saw that post here a few days ago! How could I forget about that? I might just switch over to Mullvad. Way cheaper, I can downgrade the Proton account I have (I'll still use their email service until I have time and figure out how to self-host my own email) and I can use Mullvads encrypted DNS servers until I can configure DNS-over-HTTPS + Unbound on my PiHole.
Outside of the (theoretical) technical specs of the OPi5 being better, I've heard/read mixed things about OPis. Some say they're a good alternative, some say they're cheap Chinese-made crap. I've had no experience myself, so take it with a grain of salt.
I'm interested to see more data on the RPi5 when it's out, as to figure out if it's worth getting over trying an OPi5 for a home media system with Jellyfin.