Every 3 days is quite a lot for a normal stocked aqaurium... Not that it is to much, after we don't know how much water you change at once.. You might have your reasons to do what you do.. Anyway once a week 50% or 60% or 70% you decide might do as well instead of every 3 days 20%..
And don't know how you syphon the dirt from the substrate but there are some gadgets to make very easily and make cleaning more suficient. For example take take a lenght of 12mm hose a small piece of 10 mm tube and a 0.5 litre pet bottle.. Drill a 12mm hole in the cap and cut off the bottles bottom.
Put the small piece of 10mm tube into the 12mm hose.. Now stick the hose through the bottle cap so you have the tube in the bottle, the hose wont go all the way through, it stops at the tube, there the hose is thicker than 12mm. Pull it tight into the cap. And you end up with this.
A bell syphon?.. Anyway, it won't syphyn very large volumes because it's only 12mm hose.. The created bell (the bottle) actualy lowers the suction speed a bit more because it's much wider. syphon with this and stick the bottle in the substrate and pull it gently up, a bit of substrate is lifted but fall immediately back again, but the dirt in it, swirls up in the bell (bottle). Once the bell is sucking clear water again do the same thing at another dirty spot.. And systematically work your way over te substrate. It wont suck hard but enough to take up the dirt, in my case it also cleans the substrate bellow the carpet without disturbing the plants. Because of the hose size you have more than a few minutes before a 10 litre bucket is full..
Awfully simple and cheap and awfully effective in cleaning substrate..