Hey there! I have goats and make a lot of chèvre so let me offer a few thoughts.
First, chèvre is primarily a lactic acid set cheese...yes a tiny bit of rennet is used--like 2 drops for a gallon. But it is intended to be a slowly ripened, and long-drained cheese. That's why you don't ripen it too fast or cut the curd and drain early. And yes it IS very acidic...with chèvre you're reaching for a pH around 4.6 or so and you should see the curd pulling away slightly from the sides of the pot and there should be about 1/2 inch of whey floating above the curd mass.
Because it is primarily lactic acid set, you need the long time (mine usually only needs 12 hours though) for the culture to work and set the curd. The rennet is just an adjunct. If you use too much rennet you can get a weird texture. But usually the main reason for a coarse (?grainy?) cheese is that it was resting at too high a temperature. How warm did you take the milk to? Many recipes say up to 86-88F degrees, but I've found that with raw goat milk you end up with a less creamy final cheese. So I raise my to only 76-78F, add the cultures and let it remain at room temp (which for me is also around 78F.) I've not made it with kefir, but I know you can; I'm just not sure how much you would need to add.