An opensource alternative to Mavic Mini

I’m looking for a drone that has the following features:

• Weighs less than 250 grams (to avoid the legal obstacles for use);
• Can resist wind up to 8 m/s;
• Can use GPS and barometer to maintain its position and altitude, so that the operator can concentrate on actual mission instead of being busy operating the craft;
• Has downward vision sensors to maintain position at smaller altitudes without GPS;
• Has a camera on a stabilizing gimbal;
• Can stream FPV video to a mobile device and take still photos;
• Can continuously fly for 30 minutes;
• Is ready to fly out of the box (i.e. no need to build it first);
• Its firmware and ground control software are free and opensource, so that I could modify them and actually fly with these modifications.

All of these items except the last one are satisfied by DJI Mavic Mini. But the last one isn’t, and I’d like to find a good alternative that satisfies all the items for a price no more than 2× that of Mavic Mini. So the price should be no more than about 800 USD.

I already have the Mini, so if the suggested option is a mod for it rather than a whole new drone (e.g. a custom mainboard), it’s also acceptable.

If the suggested option is a kit to assemble from instead of the ready to fly drone, this is also acceptable provided that the kit has everything to get the complete ready to fly drone, without any need to purchase additional parts.