Not even gonna sugarcoat it. Get a thong with very thin straps (i.e. nothing ruffly or banded, just thin elastic). If you don’t wear them on a regular basis it’ll take a while to get used to but that’s what I usually wear with any tight outfit in order to avoid panty lines.
Alternatively, get a dance bodytight. It’s okay to not wear underwear with those (although I personally find it uncomfortable) so if you don’t like how thongs feel, you can use those instead. Plus they add an extra layer of security and help the bodysuit look much more smooth, so they’re a good idea to wear with spandex anyways!