Cheap and freedom-respecting are hard to find together. But I find second-hand high-end tablets re-flashed with LineageOS are the sweet spot! See if you can find something listed as compatible on here -

Devices | LineageOS Wiki

Don't worry if its officially "End Of Life", LineageOS supports older models for many many years if they're popular, keeps them fast and secure. It is of course free and open source.

Thanks!