Rethink seems to need to use a VPN slot to work, which means I can’t use IVPN’s app. I guess this means I’ll have to use Rethink with NextDNS inside the app and IVPN as a Wireguard profile?
You need to download an IVPN wireguard config file and import it via the proxy section in rethinkDNS.
If you really wanna use the app together with RethinkDNS and not just the VPN connection, there is a way using sing-box. See my post here: Using VPN + DNS on android? - #20 by carbonated (point 2)
But via Wireguard is the standard way to go.