[Sparktank]

for HDR -> SDR you can use FFMPEG's internal tone-mapping.
https://ffmpeg.org/ffmpeg-filters.html#tonemap
You'll have test various parts to ensure that different scenes don't look too out of place (light vs dark vs natural light vs etc).

It also looks pale if it's HDR content, which will need to be converted to SDR (via FFMPEG).

You could encode to HEVC in 1080p and keep the HDR metadata to passthru and let MadVR convert to SDR (this limits to PC playback only).

There's a vapoursynth script somewhere here to convert HDR -> SDR. Not sure how well it works compared to FFMPEG conversions.
https://forum.doom9.org/showthread.php?t=174415

Those are the only automatic processes that are free and easily accessible.