How do I get softer peanut butter cup ice cream?
I believe your base has a butterfat of 11.6%, which is fairly low, at least for home recipes that're usually closer to 19%. 

I agree that it seems like the fat in the peanut butter should offset that, but maybe it's not. Maybe the low butterfat content is causing the peanut butter to essentially freeze the same way it would if you just put the jar in the freezer, and you actually need more butterfat to emulsify with the peanut butter to help prevent it from freezing so hard.

Obviously, this is a guess, but otherwise the amount and type of peanut butter you're adding seems fine compared to other recipes. Also, you should be able to skip the guar gum with all that peanut butter, especially if you increase the butterfat.

You might try changing the base to be 400 ml double cream and 475 ml whole milk, which would be the same amount of liquid but with a butterfat of 18.6%.

Ice Cream Butterfat Calculator