As far as the DJI alternatives, if all you need is basic thermal or zoom, up to intergrating LiDAR or UV detectors with long flight times, dual/ triple sensors and feeds, more robust signal to counter interference or noise, or maybe an energy efficient mono wing airplane for mapping or search and rescue missions, high accuracy gps integration for surveying - basically, anything you need very specific features for, you don't have to spend a ridiculous amount of money for dji rigs, which are very versatile. But you can put together a ready to fly bird and a back-up and have a lot more batteries/ charging stations for the price of one high end dji setup with limited peripherals. Using autopilots like pixhawk 2 with the cube that can do a lot of onboard processing is pretty powerful. Building your own rig also means you have a lot of versatility when it comes to charging and the price of the batteries is much lower... dji is great and all, but the prices are awfully steep when you're just trying to do basic inspections and mapping with limited resources. Then you have the firmware nightmares, mysterious glitches, poor customer support for something you can't really dig into to fix yourself without voiding warranties.
The hard part is learning how to build what you want if you're use to dji having everything turn key with easy interfacing and not thinking twice about batteries, esc's, motors, achieving dual or preferably triple redundancy, designing airframes if necessary (mainly monowing stuff), I don't believe dji has anything to offer for airplanes as far flight controllers go.
I mean one dji radio is what, $500? No versatility with switch assignments, no flight modes other than controlling gps modes. I have a lot pet peeves when it comes to dji. In the context of videography, hands down, dji. I don't have the software knowledge to build a rig better than what the inspire 1 can do when using 3rd party apps. It's phenomenal and cheap for what you can achieve with it.
For a mostly hassle free acquisition, no brainer interface and no time spent planning, designing and building; DJI is legendary! They are the reason why it's so easy to get started.
In your case, as a side thing, no reason to burden yourself with custom setups. Dji wraps it all in a nice pretty bow. For me, I've always enjoyed building and testing. It wasn't a very big leap to go from the rc mindset to the commercial drone mindset. It is crazy how fast things have developed in the last few years in the open source world!
DJI is like Apple
Custom setups are like an android. Much more flexibility and cost efficiency
Here I go ranting about equipment again