I have Pico i.MX7 Development Kit for Android Things
https://www.nxp.com/docs/en/user-guide/PICO-IMX7UL-USG.pdf
other than Pico-PI rev b1 it has other hardware, including screen
I only need Pico-PI to use as a server for some network task. However when I turn it on it shows "android things" loading and then blank screen. (i guess because android things is deprecated)
Is there any way to uninstall "android things" completely, flash it with Raspberry Pi OS?
No, because the device itself is not a Raspberry Pi. However, the manufacturer of the Pico i.MX7 kits (TechNexion) does claim to support other OS variants (including Linux).
I would recommend checking out the Pico i.MX7 documentation for additional details on how to get started flashing the board with a different OS.