Getting a double-VPN setup to work with MTU differences?

Background: I have a cellular ISP and therefore cannot configure the CGNAT. After burning through some dumb ideas (free reverse proxy, docker) I realized I could just use my paid VPN.

My setup is as follows: on the VPN server create a tunnel to AirVPN and start the openvpn daemon. This creates tun0 and tun1 with their own 10.x.x.x/24 subnets. The home network has 192.168.12.0/24.

It's possible to troubleshoot the MTU with ping -M do -s xxxx y.y.y.y to the VPN public address and test TCP/UDP sockets with nc -l -u -p 1194 .

I'm not sure if the MTU is variable across servers, but for the server I am on now ping -M do -s 1432 x.x.x.x is the biggest I can get a response from. 1432+20+8=1460 bytes.

Regardless, connecting to the home VPN through the AirVPN link still causes breakage. Discord seems to be what isn't working, mostly. Everything else has 200 ms latency as expected and not everything pings correctly. Rarely it will tell me the MTU has to be adjusted, sometimes tells me "message too long" and mostly just ignores my ping.

Can someone give me a recommendation for what MTU to be setting in my local OpenVPN server? Should I use mssfix or tun-mtu? Should I lower the MTU of the AirVPN connection? What else can I do?

Diagram:

Home -> AirVPN <- (1460 MTU) -> OpenVPN Client & server -> (1300 MTU) -> Home -> Outside World

In the process of doing this I somehow shut my house's WiFi down...