I think DJI is pretty locked down. Probably you would want to build a new drone, OpenHD uses a Raspberry Pi and ExpressLRS uses STM32 (and a few other various chips) but you can get DIY kits that are relatively easy to set up on sites like https://www.getfpv.com for example.

Also check out INAV if you want to build a more DJI-style drone, this would be the Flight Control software: https://github.com/iNavFlight/inav

Other FC software include ArduPilot for more of a groundstation approach, while Betaflight, EmuFlight, and similar FC software are what FPV race, freestyle, and cinematic pilots prefer.

This is correct, but to clarify something, you don't need ExpressLRS for OpenHD, they're entirely independent (one is for control, the other for video).

Also, Ardupilot blows INAV completely out of the water for reliability, at least for fixed wing craft. I've switched all my planes to it from INAV.

What constitutes a failure in reliability for this type of software? The drone falling out of the sky? Flying off in some random direction for no visible reason?

In particular, INAV has problems with keeping the position of the horizon accurately, and usually thinks it's going straight while it's dramatically losing altitude. So, "all of the above", really.

The biggest issue is that this usually happens during automatic modes, which are things like "return to home", which you usually use when you've lost control or video and you really want RTH to work reliably. I can't risk a plane falling onto a car or person, so I use ArduPilot, whose auto modes are impressively accurate.