Thingino is a replacement of the stock firmware. But you obviously lose the ability to use vendor's cloud. Because if you still want to use the subscription service, why would you need to replace the stock firmware?
The cheapest supported camera, aside from a given you as a gift, is Jooan A2R from Temu. It is like $3 per piece, brand new. It is a nice little pan/tilt model with a low-end 720p sensor. But it is fast, nice looking, snappy and dirt cheap.
Hey, Openmiko is a nice project. With your baggage of knowledge, I would love to see you contributing to Thingino as well. While we still depend on binary blobs from the manufacturer SDK, there is a work on alternatives to replace what is replaceable with open stack. Join the team, have fun.