So I'm thinking of building my own drone. One key feature I would like to add is 4G/5G drone control and video transmission for long range flying. Is it possible to add 4G/5G support to DJI flight controllers or should I just make my own controller from a microcomputer like Raspberry?

Don't forget European law on civilian drones.
Orient on Pixhawk / Ardupilot for this project. You put a microcomputer in the drone which has an internet connection via a 4G key. Here you have just obtained an almost unlimited range.
I use a Zotac computer and a Kruger & Matz 4G key.