>Couldn’t it also work by guaranteeing each state X seats and then the rest Y seats are set according to census data on population?

Yes. If you call the "X" club the Senate and the "Y" club the House of Representatives, this is exactly how our bicameral legislature works.

edit: Their votes count for passage in their chamber, not equally weighted against eachother. If you mean Y seats equal seats by population but with a minimum X, then that's how the House works. Any proposal to make the senate proportional starts to ask why we're not unicameral because then you basically have 2x house of reps but with different voting district sizes.