True fully-open WiFi camera hardware is basically nonexistent. Closest you'll get is OpenIPC (openipc.org), open source firmware for the SoCs a bunch of budget cameras use, but it's a compatibility gamble per exact model, not plug-and-play.

More realistic path for a Tapo-style setup: keep the camera on an isolated VLAN with zero internet route (kills the cloud phone-home even on stock firmware), point it at a self-hosted NVR like Frigate or Shinobi over local RTSP/ONVIF, and never touch the TP-Link app or cloud account. Doesn't make the firmware itself open, but it makes the actual footage 100% local, which is usually the part people actually care about.

If you specifically want reflashable open firmware, OpenIPC's supported-devices list is worth checking before buying anything new.