@christopherk First of all, your milk is tailored for the needs of your baby, therefore it can’t be “too thin”. Second, if baby only ‘needs’ more when grandma is looking after him, there’s your answer, grandma is over feeding. The only thing you can do is give her the prepped bottles with a schedule and stop worrying about it. Soon he will start on solids and his milk consumption will decrease slowly, no point on your trying to increase your supply when the demand is about to go down.
My baby used to drink 12oz in 9h at Daycare at 6 months. Now he is 8 months and eating 3 meals a day, his consumption is down to 4oz in 9h. He still breastfeeds on demands at 6pm, 8pm, 11pm, 3am, 6am. We are trying to wean the middle of the night feeds but he usually eats in 5 mins and goes back to sleep on his own.
You can top off with Formula if it helps with the stress but I wouldn’t recommend it, not because there’s anything wrong with Formula but because over feeding a baby is not good.