I don't own either of your devices, but since nobody else has commented I can at least answer your question about ARC. Sending ARC from the TV to the 975 would give you at best the same audio as using SPDIF/coax (compressed Dolby Digital, not TrueHD). Some TVs don't handle passing multichannel from an HDMI input source well.
You may have already done this, but there is a setting in System Settings to direct HDMI Audio Output to HDMI2 (instead of Auto).
Outlaw Audio Model 976 / 7700 / Ultra-X13
B&W CM9, CM Center, DS3 (side), DM303 (back)
Blue Jeans Cables (XLRs, Canare 4S11 Bi-wire, HDMI)
Tivo Bolt, Xbox Series X, Panasonic ub820, AppleTV 4k, LG C765P
Roon, Allo Digione Signature, Sonore MicroRendu, Uptone Audio LPS-1, Peachtree Audio Nova 150
GIK Acoustics Soffit Bass Traps, 244 Panels, Monster Bass Traps, QRD7 / Polyfusor Diffusers