What I use for both carbines and shotguns is something like this-
start with pistol mags downloaded to 3-5 rounds, carbine mags just a few rounds, 12ga two rounds.
Shoot the shotgun until empty (insert carbine if that is what you are using), transition to side arm. shoot until slide-lock, speed reload, scan, holster, recover long gun, speed reload 2 shells of 'gauge or a short magazine, and you are reset for another rotation.
Having only perused the thread, I will add only to learn as many ways to reload as you can.
Im a righty:
The "workspace" reload works good for open spaces or movement, where a safe direction is up.
Muzzle down reload I use where is it unsafe to go above horizontal with the muzzle- ie our agency's ranges, both indoor and outdoor.
Shotguns are big and heavy, and I have found trying to reload while covering an area, person is a total pain unless I have something to prop the gun against. My personal opinion, at this point some tactics/comms come into play, and simply ask someone, if available, to cover your object while you take 15 seconds to top off.
Flipping the gun on its back and loading that way has its place. The thing to bear in mind, is your left hand is controlling an out-of-play shotgun (you have lost fire control). Should the necessity arise, drop the shell youre holding, grab your gat and fix the problem.
What works for me is to load using whatever side hand the ammo is on. Mostly that works out to be my left, however, SpeedFeed stocks/stock sleeves put some ammo on the right.