Retarding enough to make a full throttle shift possible is going to dump a massive amount of heat into the exhaust valve area and if the engine is working hard (I guess it is since you're considering full throttle shifts) this is a bad idea. There are pros and cons to fuel cut and spark cut, but if you're running carbs fuel cut is not practical anyway so I'd recommend spark cut. I made a full throttle gear shift controller in hardware using a shift light (for rpm sensing) and a couple of relays and a clutch switch a couple of decades ago but I could never persuade myself to trust it enough to actually use it in anger.