Using Canon's Digital Photo Professional 4 to open your raw file, it seems your camera used the Standard picture style, a White Balance setting of around 3800K, no tint corrections, and a fairly neutral contrast curve (neither high contrast nor low contrast). The only lens correction was for 'Peripheral illumination' set at 70%.

Any attempt to flatten the highlights or boost the shadows will very quickly result in the banding you're seeing in your example above. It looks like you tried to compress the dynamic range with ART and that's what caused such severe banding.

There are a few areas where the highlights are blown, but any brute force attempt to pull them back will result in the mid highs also being flattened, and that's the major source of your banding. Pulling exposure/brightness back requires a 2/3 stop adjustment to eliminate the brightest highlights from blowing out.

Here's what I got with Canon's DPP 4 using the default in-camera settings except for changing WB from 'Auto - ambience priority' to '3800K'.

By pushing brightness by 1/6 stop, changing the WB to 4600K with a slight tint towards magenta, crushing the shadows just a bit (to deal with a little noise in the very dark shadows, reducing contrast very slightly, and using the HSL tool to adjust individual colors (more on that below), I got the following result.

I also applied lens correction at the default settings for CA, color blur, and distortion.

Here are screenshots of the two key tabs in Canon's Digital Photo Professional 4 with the adjustments I used.

The HSL adjustments in the red, orange, and yellow bands pulled the red and yellow areas towards orange, which is the predominant color in the photo. This smoothed the transitions between each color band. There is virtually no green, aqua, blue, purple, or magenta in the image, so we made no adjustments at all to those color bands. By reducing overall saturation on the first tab and then pushing it in the HSL tool, it made the effects of the individual color channel adjustments stronger.