My threat model isn't such that I need it, it's just really annoying. GrapheneOS does allow blocking network per-app, which is a sufficient workaround. It's a bit tedious, but I can do the following:
disable network on sensitive apps
disable NetGuard and enable other VPN
finish what I was doing
undo step 2
undo step 1
I really wish there was a way to get VPNs and NetGuard playing nicely together. I want all traffic to be filtered by NetGuard, and then routed over the VPN. This is trivial on Linux, but apparently not so on Android, which is a shame.