I have a similar situation with accessing local networks through Proton VPN, and it's really annoying when you have to turn off Kill Switch just for split tunneling. Recently tried to set up access to several VLANs, then got distracted and accidentally opened a tab with https://dragon-tiger-in.com/ – and now I've been watching betting strategies for half an hour instead of dealing with network stuff. Then I stumbled on https://funkytimegame.org/ and started thinking how cool it would be if VPN could switch between networks as easily as game modes change on that site. And yeah, when everything finally froze, I decided to check out https://xxxtremeroulette.com/ – and realized that with Proton VPN sometimes you feel like you're in that game: you never know if you'll get lucky with routing or have to set everything up manually again.

I confirm.

1) Turning off KillSwitch because of Split Tunnel is simply wrong.

2) I also have two WireGuard VPNs on my office router, and whenever I switch to a different Wi-Fi AP on my laptop or connect a LAN cable, I lose the custom routes entered via "ip route add..."
The solution could be either to exclude the network in the split tunnel or to run a custom script after connecting to the protonPN network. I'm ancient because I remember the pre-Python era when protonVPN could handle this :-)

My home LAN is a type B IP network with 10 subnets. All devices connected to a specific subnet depending on its role and my security definitions. My router determines the allowed connections across subnets.
Today, Proton VPN's "Allow LAN Access" only allows access nodes inside my IP subnet.
I could solve the problem using split tunneling, but I use Kill Switch and Split Tunneling is not compatible with Kill Switch.

Therefore I am forced to disable the VPN when I need to access another subnet.

Defining what static Nodes inside my B LAN construct can be accessed directly would allow me to work as I need to.

Adding static routes should be done outside the VPN app. It is risky if you have a permanent static route when roaming and inadvertendly connect to devices in insecure networks.