Craig
Were you shooting through glass such as the windscreen? If so then your problem is mainly the optical effect of the glass. Auto glass is designed for safety not clarity or light transmission and windscreens are particularly problematic.
Also, did you have the car engine running whilst shooting from in the car? The vibrations from car engines are very problematic for modern high megapixel cameras and long telephotos. On a recent trip to africa I was usually shooting from a vehicle and managing this vibration issue was critical to sharp shots. The vibrations may also interfere with the image stabilisation given that it involves moving a lens element or sensor at very high frequency, but not sure about that.
I have recently experienced the problem of heat haze both in my 2023 trip to Africa (very hot, approaching 40 degrees on some days in some locations) and recently on a trip to Fivebough swamp in Leeton (not excessively hot, but clearly a hot of heat reflection off the water and reeds. Heat haze in these enviroments takes the form of wavy interference which, at the extreme, leaves objects beyond 10-20 m with a weird wavy effect (happy to share examples as I still have the raw files). The photos are not just soft, they are completely unusable as distant objects are quite distorted.
As you haven't shared the exif data then we can't comment on whether settings might have had any effect. Also things like UV filters can affect sharpness (and bokeh), if you use them. I guess what I am implying is that sharpness problems could be from any one or more of several sources to do with camera and lens settings, camera movement, lens/body stabilisation control, or environmental issues, and all need to be considered and eliminated.
Simon