Oh wow, if you're getting a regulator and tank that'll be great! About the chlorine/chloramine, no, its a common misunderstanding that they are like some sort of poison that could damage things immediately but that's not the case, they are slow-acting and any lag time even if you dosed the conditioner after the fill would likely cause no harm at all. On the other hand it only makes sense to dose some beforehand, I dose half mine right before and half directly afterwards but its just a game I play that I think helps me not ever forget it altogether!
Chlorine/chloramine are dosed in municipal water systems at basically a minimal amount to discourage bacterial growth *within* the pipes. The dosage is not something that typically has a powerful or lasting effect after the water is out of the pipes, although of course chloramine hangs around much longer than chlorine.
Its your own choice, 6 months?, 1 year? (those would be my typical two breakpoints personally) for the period to consider the colonies to be not yet fully mature and thus worth spending some extra pennies on for lower risk by dosing at 1.5x to 2x (but not more than 2x)... but at one of those breakpoints I'd probably drop back to 1x. And yes, when you direct fill, you always dose to the full tank volume no matter what percentage the water change is. I recommend Seachem Prime for that first year and then you could compare its price per treated volume with whatever other concentrated pond dechlor conditioner you could find (it just takes carrying a pencil and paper to the LFS or using your phone perhaps, to do out the calculations while looking at the dosing instructions on the pond conditioners.)
~~waterdrop~~