AFAIK, there are only 2 games currently that can only be played on Series X/S/PC. Scorn and Flight Simulator. The Medium and Batman Gothic Knights are available PS5. Jedi Survivor will also be on PS5.

Starfield and Redfall will likely not be on last gen but neither of those have release dates. I think you have a good 6 months to a year to save up for a Series X (or PC) if these titles are important. That's assuming they don't get delayed. The longer you wait, the better.