Here’s my quick review of Xiaomi Mi 9 running CrDroid GSI Android 10 ROM. Really fast performance, excellent battery life, and Magisk SafetyNet check passes with working Google Pay. If you want a really good old phone, I think Xiaomi Mi 9 is a great choice if you install a GSI ROM like this. My current setup here runs waaay faster than my Samsung Galaxy Note 20 Ultra w/ Samsung stock firmware. If you want pure Google experience w/ low-cost hardware, Xiaomi phones are where it’s at since they allow you to fully unlock the bootloader and the stock MIUI firmware is nearly 100% compatible with Android AOSP.
For full description of CrDroid GSI, see on our forum.