You can set a save path for mods but there's no way to change the base game default save location AFAIK. You could just run a "mod" with no changes with a custom save path and that'll still work. Watch this video from 0:32 to 1:30 or so, he explains how to set up mod save paths. I think this is most likely the best solution for you. HmGUwy.

diablo 2 resurrected offline save location