I'm totally out of the loop with build it yourself drones, but I'm looking for a drone project to build with my 12-year-old son since he's at home and out of school now.
Would like to find something that uses mostly off-the-shelf parts and open-source software and electronics. We have a DJI Mavic Mini, and would love to build my own with similar capabilities... Return to home, GPS, take pictures, long range video, etc.

Check out GetFPV.com
https://www.getfpv.com/

Or ReadyMadeRc.com
https://www.readymaderc.com/

Or if you are interested in a fixed wing/flying wing for FPV, then Ritewing.
http://ritewingrc.com/product/ritewing-drak-mini-kit-2/
(There are a couple other designs of forms of flying wings from Ritewing, besides the mini Drak)

If you want return to home, position hold, and even some mission/waypoint flying then you need to look into Inav.

There are specific boards they support and firmware targets.   Those are the flight controllers you will want to focus on.

When you grab a controller download steam and get the controller working with something like the Drone Racing League and start practicing FPV.   If you are gonna scratch build a drone you are 90% of the way to being in FPV anyhow and you will feel as if the shackles were rem9ved when you start flying FPV in acro.

You can build some small FPV rigs on Inav that will still be GPS capable.   You generally need two UARTs.  These will be where your devices like RC control link and the GPS receiver connect.

I am currently building a palm sized sub 250g FPV drone for long range flying that has a 1080p camera(4k is possible) and should be able to fly out to nearly a mile to mile and a half and back.