This highlights some of the problems. In fact it will not take longer for calcium and kh to drop, all else being equal they will still drop at the same rate as they precipitate, and that will mean so much more to replace. PH will also drop the same, reason being the extra water is going to be unlit and not productive in terms of eliminating any Co2 via photosynthesis. However nitrate buildup will be diluted by the larger water quantity.
To me, a decent sized sump is a good thing to allow work & equipment room, but it is largely a fallacy that heaps of extra water is going to be a great benefit. The idea was popular many years ago when tanks were less stable and disasters more common. A big body of water was a buffer to lessen the impact of these problems. However nowadays it's less needed. If installing a big body of water just to "have" it, have to measure benefit against cost, and there will be a cost. That cost may have been able to be better utilised.