I can’t wrap my head around what you say about the TV speakers rivaling your audio setup. If you can’t hear the difference, something is seriously amiss.
Try using a real glass Toslink in place of the eARC and see if you hear an improvement. Technics refused to use an HDMI input on their SU-R1000 and SU-G700M2 because they couldn’t figure out a way to stop video noise from contaminating the Audio out with eARC. Apparently, that video noise is present in every HDMI audio out if it’s coming from a TV.
No matter how nice new TVs look, they are not audiophile quality. (just waiting for someone with a scope to scream "look, it’s perfect!")
Technics finally figured it out and it’s now used on their SU-GX70 integrated/streamer. Somehow, they can now properly block it at the input. The minds behind Technics are operating on another level and I thank Panasonic for bringing them back.
All the best,