If you want a mavic, just buy a mavic, building something with this level quality / reliability is not worth the effort.

The advantage to building your own is that you get a different type of quad -- a racing quad which feels more like flying a porsche (feels like a race car) than an mini-van (e.g. mavic, reliable but super fun to fly).

If you want to build a racer, rctimer has some of the best kits (high quality parts, well thought out design). Here's an old build guide from my blog, you would want to get a newer kit with an omnibus-based flight controller, but the idea is the same: https://nathan.vertile.com/blog/2016/04/24/rctimer-u210-mini...

Also, if you're just learning to fly, you probably want to learn acro/rate mode (which is full manual mode) on something smaller first, like this maybe: https://nathan.vertile.com/blog/2017/03/31/betaflight-micro-...

I'd recommend going to the youtube Channels Rotor Riot and UAV Futures (and possibly Mr. Steele) if you want to get a nice series of well-sourced introductory materials by veteran professional drone operators.

I'm by far an amateur, and don't really have enough sim hours to fly the drone I'm just about done assembling.

But: the aerial camera gimbal is by far the hardest part to get right, and why vendors like DJI charge what they charge. Software exists to make it work, though.