Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

My Radarr alone uses about 250 MB of RAM idle. Considering Pi Zero has 512MB, I don't think it's going to work that well with Radarr alone let alone radarr and sonarr..

A Zero would probably be way underpowered for the job. I've used a Pi 4 in the past and it worked ok, but choked occasionally.

My actual recommendation would be a small x86 box, something like a Lenovo Thinkcentre Tiny. You can get them used for about the price of a Pi, and they'll be much more reliable.

Another option you could consider is rather than running the VPN on the pi and downloading through that, to run the VPN on your media server as you planned and instead use the pi (or your router if you can run openwrt) as the remote access point. Then you only need to worry about the performance needed for remote access

I'm not sure how tailscale works, but this is what I do with zerotier (i.e. run it on my router).

pi zero is way too weak tbh, i think cpu will be 100% all the time.

Personally, I’d expect it to be fine. There’s not a lot of CPU usage for those apps. The only part I see it potentially struggling is the file transfer (WiFi on the Zero W or Ethernet-over-USB).

Might be better to get a Pi 3b with Ethernet on board

raspi 4 2gb here. Works just fine. Don't know what people are talking about being slow etc. Look into dietpi. I'm running radarr, sonarr, nzbget, qbitorrent, kodi and mullvad vpn. No issues here whatsoever.

I think not, because sonarr and radarr webUI is pretty slow on our machine, which definitely is not weak at all (we have server grade CPU and over 100gb of RAM. RAM is the biggest bottleneck for us right now, but we have to upgrade CPU in order to be able to handle more RAM.