Hello! I switched Dylan from breastmilk to soymilk for his daytime feeds around 16mo. I used much the same method as you, and at about the same rate TBH! I was going that slowly because we were having problems with rejection on full soy, so i thought a gradual switch would be better. I agree that you do have to be careful with the pumping so not to cause an oversupply, but you can gradually wean the pump too (just gradually pump for less time).
Re: the constipation though, at this age you do have more options. Even little things in the diet (lots of fruits, especially pears, peaches, prunes, apricots, not bananas) and ensuring plenty of water can help. Going gradually will do no harm at all, but if you want to move at a faster pace an oz every couple of days will probably be slow enough for your lo to adjust.
Hth!