Okay... my thoughts:
First off, leave the pH meter turned off the next time you do this. there is no reason to fly instruments before you figure out to take off and land... get the feeling - look, smell, touch of the right curd and memorize it. Chevre has been done for 1000 years without pH meter by poor farmers. You can trust the process and do it blindly. Just remember that the goat's feed is a bit different in the different seasons and if your room is hot you can adjust the ripening time down, or if you over-heated your milk you shouldn't wait many hours; you know, common sense things. You seem to be already on the ball with remembering to avoid over acidification.
Secondly, do not touch the milk/curd until it's time (see below)
You pasteurized the milk - is this your own milk? You said you pasteurized at 145F for 30 min and then you said you heated it to 83F. I assume you cooled down the milk in between? Did you cool it down rapidly after pasteurization? If not, you might have continued to cook it by letting it cool on its own and over-pasteurized it which would make it more difficult for curd to set. It is better if you cool it rapidly after pasteurization to your desire make temperature and start making the cheese right then and there.
Another reason for your soft curd could be that you have pasteurized your milk but didn't add CalCl2 to it, though lactic/semi-lactic Chèvre curds are never really set hard. It's not a clean break kind of curd but a bit more loose and gentle.
So, to adjust your recipe I would go semi-lactic here (whether you do a simple Chèvre or an aged surface ripened version like a Crottin):
- Milk: heat to 76F-78F
- Rennet: Reduce to 1/16th teaspoon and add immediately (no ripening!!!) with the MM100.
- If you want to surface ripen it as a Crottin, Saint Maure, Valençay, Cabecou Etc. you can add to the rennet and MM100 also:
- 1/8 teaspoon Geo (I suggest Danisco Geo 15)
- 1/8 teaspoon PC (I suggest Danisco PC-VS)
- Optional: To give it the traditional grassy funky aroma and better color, add 1/8 teaspoon Danisco PLA. It does contain Geo so you may want to adjust your Geo down a bit
- Ripening/Setting: Close lid, don't disturb (no pH or Temperature checks, no moving the pot, no checking for break) for 12 hours.
You can adjust it to 8 or 10 if your room is warm and up to 18 if it's very cold. By the 8th hour you can open the lid to look and smell it already. you will see that the curd has set when you recognize its pulling away from the sides of the pot and you have whey all around it. I finish it when it starts smelling too much like a Greek yogurt. (Unless you want it more tangy, then you can wait longer)
- Curd Cutting: Skip that, no need for that with this kind of soft wet curd.
- Draining/Ladling: If you have Chèvre molds - use them. The problem with cheese cloth is that it usually leaves lots of curd in the middle that hasn't drained properly. It also smashes your curd, which is OK if you want to consume the Chèvre right away or make yogurt or Fromage Blanc, but to get a good Chèvre, just ladle it into molds and wait. Ladle gently into molds in thin layers.
- Waiting in the molds: give it time. Long time. if it looks dry enugh to turn at 18 hours than you can try but no urgent need to do so, you can wait 24 or 36 hours. drain it, drain it, drain it. If your room is warm and you don't want more acidification while you wait for your cheese to drain, then continue to drain it by placing the molds on their draining pan inside your cool cave instead of out on the counter. (But not in refrigerator as it will harden the fats in the cheese and arrest the drainage)
- Once you are able to remove them, salt them immediately and keep draining them for another 24 hours. The salt will do its work to remove whey aggressively out of the cheese. Salt them with roughly 2.5%-3% of the cheese weight in salt (Use non-iodized salt or kosher salt without anti-cacking agents in it). It would seem like a lot of salt but so much whey will come out that it will remove a lot of the salt out.
- Aging: If you want to age it or surface ripen it than only now, (the third day or so) you can stop draining and begin aging. Good aging temp is 55F. Start by partially covering the box and wiping it mornings+evenings. Turn the cheese once a day if you can hold it (It will be soft and slimy for the first week or so). As you see less and less water beads, you can close the lid further to keep it at 90%RH. No need for spray. Mold will show up in about a week. If you use the more aggressive Geo 17 and PC Neige from Danisco the mold will show up in 2-3 days but would have different qualities to it.
A couple of aging tips:
1. It is ok to pack the cheese a bit tighter. After it has developed a rind, tap it with your finger when you turn it. The tapping will pack it tighter, remove unwanted air and whey and your finger will help carry bacteria to contaminate the cheese with its own mold on all sides
2. If you can get hay (French cheese shop hay is great) use it to age the cheese instead of ripening mat. It will wick out moisture incredibly well and will give the cheese this fantastic grassy rustic aroma. It also helps encourage desirable bacterial growth and correct rind pH level.
This should be perfect to eat at 14 days but can be consumed before that. After day 21 it will begin to dry and turn harder. It takes a whole new character. Blue and brown mold spotting after 21-28 days are normal and the cheese can be consumed with them. It eventually wraps itself in blue mold that turns dark and the cheese looks like horse dropping - hence the name Crottin (Dropping or Dung in French).
Two people on this board that have excellent experience with this particular type of cheese which I learned a lot from are Alex and Francois. Alex typically use buttermilk as a starter. Francois is a professional cheesemaker with more accurate and scientific knowledge than I'll ever have. That being said, I guarantee you that if you follow the steps I mentioned here you will get a great Crottin. No skin slip or over-ripening.