Has anyone ever played the 1992 game Star Legions? I remember it being this insanely fun game about planetary invasion. Does anyone know where I could get it or a similar game that is more modern?

The premise of the game was a lot of fun I am surprised that, to my knowledge, no other game has tried to mimic the theme

Congratulation. You remembered about the game when the developer's page where the entire Star Fleet series and Star Legions were available for free got shut down for some reason.

Does anyone know about any other games like star legions or any other planetary invasion PC strategy games out there?

Specifically planetary invasions - nope. 4X strategies usually simplify ground battles to statistics. However SSI Star General had planetary invasions implemented as a part of it's gameplay: you had to destroy planet's defenses, pull up the dropships and march your squads across the lands, possibly with orbital fire support.