I decided to rebuild my first ever quad this summer. The thing was built on a budget and flew like a boat. The receiver was huge (stock FlySky 8ch) and the Flight Controller, ESC and PDB stack was a mess.
The FlySky receiver was replaced by a FrSky XM+ (literally an eighth the size), the Skyline FC and Matek PDB were replaced by a D-Link F4 (which has a PDB built-in) and lastly, thanks to the space saved by those upgrades, I was able to drop the top plate by 5mm, making the whole thing look a lot more streamline. I even bought a cheap cloverleaf antenna to keep it compact. Oh, and a Runcam Micro v2 camera. Basically, only the motors and ESC remaind the same. You can check the parts on RotorBuilds: https://rotorbuilds.com/build/5872