Sound output through monitor on Mac too loud and unadjustable
You can use a 3rd party utility called SoundSource which lets you control the volume of the external display connected to your Mac using the Macs built-in volume controls.
SoundSource is a macOS app from Rogue Amoeba, a well known and reputed developer of audio utilities for macOS.
While SoundSource is a paid app, Rogue Amoeba provides a fully functional but time-limited demo of the app which you can try to see if that resolves your issue.

P.S.: No affiliation whatsoever with Rogue Amoeba or any affiliates. I have been using SoundSource on my personal Macs and I am highly satisfied with it.