I can see both sides of this one, if you don't have much capacity you can make more beer or stronger beer by adding a can to the mash liquor. But the can will reduce the quality of what otherwise be an all grain beer.
If the quantity in the FV is around 23 litres for a normal kit size then there should be enough bitterness from the kit. If the kit bitterness is enough for you. The mash is replacing kit enhancer/DME/LME/1 kilo sugar etc.
If it was me and I could only mash 3kg of grain, and I wanted 23 litres or more, I would boost the OG with DME or LME. Then boil hops for 60 mins for bitterness and add late additions for flavour and aroma. A partial mash in other words. You will make better beer and suddenly be involved in making your own beers from recipes or from your own ideas.
But if you like that kit and just want to boost it with a mash instead of other additives, and boost the hop aroma, then your idea looks fine. I just think people should break away from kits and realise it's not hard to make beer without them, and the quality will generally be much higher.
Question, though. If it "always lacked that beer taste", how could it be an excellent brew? Surely 'that beer taste' is kind of crucial?!