What GL firmware are you using? I'm guessing the latest but I don't run a Flint v2. The v4.8.x series introduced a policy based routing feature that should solve any split tunneling needs within the 'VPN Dashboard.' Note it's not (LuCI) backup backwards-compatible with v4.7.x & earlier due to the breaking behavior change for dnsmasq. Also note GL PBR is limited to five tunnels.
That said you have a Flint v2: at the risk of putting words in your mouth you should find this entire thread of extreme interest but I'll throw you into it here: