The solution I have found for this is to go into Settings > Graphics Settings (Windows settings, not Radeon), and I can set individual .exe's for programs to run as "High Performance", which means they will always use the RX 540 when running instead of the onboard graphics. A bit tedious, but it's working.
Still feels weird that there's literally no options for this anywhere in the Radeon settings app, it feels like this should be exactly what that app is used for!