Device page shows same flash chip fitted to a R4AG purchased in March 2022, reported to be successfully flashed with OpenWrt ?
https://openwrt.org/inbox/toh/xiaomi/xiaomi_mi_router_4a_gigabit_edition#photos
According to commit
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=17e690017d76c49070bc99d6a376b1926259c5ff
GD25Q128C is supported.