First thing first: check your phone compatibility with Fitbit:

https://www.fitbit.com/us/technology/compatible-devices

From your requirements:

Versa Lite is the cheaper Fitbit Device that has Fitbit OS operating system (if Smart Wake and some apps are more important). It has exercises but with Connected GPS (GPS from phone).

Fitbit Charge 4 is the cheaper Fitbit Device with Integrated GPS (if integrated GPS is the most important factor).

Fitbit Ionic has Fitbit OS and integrated GPS (both advantages). Maybe slightly more expensive.

Sleep tracking is equally very good on both devices. All three above have SPO2 variation graph.