I hardly ever use the backpack but it does come in handy, its all i used to use, but my round isnt compact, its big ones dotted here and there, so i would just fill up when i got out of van.
400 litre tank is perfect mate
Also the pump on the backpack is amazing, ive used it at 60 ft on setting 3, (theres 4 settings) amazing little thing, had it for 4 or so years now and only had to replace the switch, the pumps still chugging away
Whatever your happy with is what counts, im just saying 400l tank is just so simple to use with a hose. Lifting trolleys in and out day in day out wont do your back any good, so just be careful.