Hey Pug. Easifill is a 2 coat jointing system, ideal for small jobs. It's used for the fill coat and finish coat. The only problem with it is that it retains moisture more so than joint filler, so if you 2 coat it in quick succession, it will take longer to thoroughly dry ready for sanding. If you put the material on tight and neat enough, there is no need to sand between coats, maybe just a quick de-nib, if necessary. Forget the 20 minute stuff, that is really only for patching and you'll be forever mixing and cleaning out your gear. Standard Easifill has a 90 minute set time and you should pay around a tenner a bag for it plus vat.
If you go the other route of joint filler and joint cement, don't use the readymixed Gyproc in the yellow and white tub as it sets like flint and is an utter t**t to rub down. You can use the jointing sponge if you wish but unless the compound you are using is set adequately, the sponge will cause it to 'pick up' and you can easily drag it off. Whatever you do, it's 2-3 visits.
The other problem with trying to put multiple coats on in the same day is that the subsequent coats may develop air holes which are hard to get rid of. If it's just one room, then use the Easifill, if it's 3 or 4, use joint filler for the first coat and bagged joint cement for the other 2, going a couple of inches wider on each coat. Paper tape your internals and scrim the tapered and butt joints.