Are you in party chat or game chat? If you're in party chat then adjusting the mic volume in the game is pointless.
I had similar issues with chat volume when switching from mixamp on PS4 to PS5 but it's because the PS4 had individual settings for game volume and chat volume, whereas on PS5 it has been combined into one balance slider, meaning it's a pain to adjust because either my party is too loud/quiet or my game is too loud/quiet. You can exit party chat and just chat in the game but that then has its own drawbacks.
In terms of the audio quality so long as your PS5 is set to send dolby bitstream via HDMI, and your TV is set to send audio via optical, passthrough, bitstream - it should sound pretty much how it did on PS4, as the mixamp is receiving the same data it would have on PS4