Da Flex, I'm still waiting for a new HDMI S/PDIF Splitter to come from interstate after the one they have here was incomplete. However..I remembered a few years ago I bought a Hat. It was for the Rpi3 but it should work on the Rpi4. It's a JustBoom Digi HAT and has S/PFID and digital audio output. If I can get this working it might be a better solution. I just need to find an overlay for it. I'll try adding dtoverlay=justboom-dac.
After dev talk I can say that the audio pass-through is independent of the target device. All available audio channels go out, including LFE.
The HDMI-1 port should send audio without any settings (user reports somewhere on this forum). Because the splitter is very likely the culprit, you don't need HDMI-1. Just replace the splitter.