The art style was very reminiscent of Speed Racer, at least as far as the era was concerned. It was either in raw Japanese or fan-subbed at the time(this was still in the mostly VHS-era, would have been 2002/2003 that I saw it but it looked old.
I may be confusing this part with another anime we watched that day, but I believe a plot-point involved salvaging ancient mecha from a precursor civilization. If I remember correctly, the people were at a roughly 1940s-level of technology, but we able to salvage and repair mecha from a more advanced era. I know that plot shows up in many different anime. I want to say most were roughly gundam-sized, then one side managed to get a skyscraper sized one working, then the other side trotted out the volcano tank.
I'm sorry I'm not able to remember more details, and thank you kindly for trying to help! This image has been haunting me for a very long while.