I hear you, I have a Legion laptop with a GTX 1060 mobile and I keep the dGPU as primary all the time because I just can't be bothered by NVIDIA optimus anymore. That's the reason I decided to upgrade to AMD, even though the performance of the 1060 was still appropriate for me and I wouldn't have upgraded yet otherwise.
I don't have any issues with the Strix G15 on Fedora Silverblue. Talking to other owners of the same model and also other Asus AMD laptops on Reddit, I didn't hear any complaints about that.
The G15 has the HDMI port connected to the iGPU, and the USB-C (DisplayPort Alt Mode) connected directly to the dGPU. I've only used HDMI to connect to a TV, I haven't tested the USB-C output because I don't have a monitor with DisplayPort. So I can't really answer your question.
Tell you what, I've just ordered a USB-C to HDMI adapter, as soon as it arrives I'll test the output that's connected directly to the dGPU and update you on that. I'd bet on it being plug-and-play, but we'll see. 😉