1. Get a wireguard config, it has a private key and peer information
2. Add a new wireguard interface, I called it proton1. Use the private key from the config file.
3. Add ProtonVPN as peer, to interface proton1, use pubkey from config file, set up IP in endpoint, and port in port. Allowed address 0.0.0.0/0.
4. In IP > Address add IP 10.2.0.2/30 to interface proton1
5. Make sure this VLAN does not use internal DNS ranges (IP -> DHCP Server). Okay, now we need to get stuff to route into that actual interface.
6. In Routing -> Tables add a new table vpntable (fib=yes).
7. In Routing -> Rules add a new policy, source address is your guest VLAN IP range, destination 0.0.0.0/0, interface guest interface. Action lookup only in table. Table vpntable.
8. In Firewall -> Mangle add a mangle rule, chain prerouting, source address VLAN IP range, action mark routing, use routing mark vpntable.
9. Almost done, you just need a masquerade rule. In Firewall -> NAT, add a new NAT rule on chain srcnat, out interface proton1. Action masquerade.

That should work.

Relevant config (first part (wireguard interface) does not have privkey, remember to set it!)

/interface wireguard
add listen-port=13231 mtu=1420 name=proton1
/interface wireguard peers
add allowed-address=0.0.0.0/0 endpoint-address=x.x.x.x endpoint-port=\
    51820 interface=proton1 public-key=\
    "xxx="
/routing table
add fib name=vpntable
/ip address
add address=10.2.0.2/30 interface=proton1 network=10.2.0.0
/ip firewall mangle
add action=mark-routing chain=prerouting log-prefix=MANGLEVPN \
    new-routing-mark=vpntable passthrough=yes src-address=10.0.80.0/24
add action=masquerade chain=srcnat comment="NAT Proton" out-interface=proton1