This isn’t exhaustion, being too busy or anything else - this is intentional & incredibly rude. It’s covert but passive aggressive, so that you cannot really say or do anything, because she’s meeting basic criteria of being clipped & polite at handovers. I feel for you esp with the amount of time & energy you’re putting in to raise the DC for her & your DS.
Your DIL sounds like she knows the difference between good & bad manners (if she’s successful in her career, is v disciplined around the DC’s routines & diets & everything else. To not know that it is good manners to thank & show gratitude to the person that’s enabling their family to function & not only that, her DC to have a loving and supportive GM - doesn’t really fit here).
Yes there is OP’s son who refers to his DW as The Boss which tells a lot. Yes he could be using that old trope to exempt himself from family duties but I suspect if he started posting thank yous of the DC with heir gifts, DIL would not be happy.
W’ever her reasons, jealousy, insecurity, territoriality - don’t waste time trying to understand her behaviour because regardless of any of those reasons - her actions are not warranted by it.
I don’t blame you Op for being worried that if you make any requests no matter how tiny, she can withdraw your access to your DGC at her choosing. It’s a horrible & precarious bind so many GP’s find themselves in. I would just be warned that as soon as you are no longer required to be their childcare, your DC will ice you out even further. Which can be extremely painful.
I would start a plan of more boundaried contact on your part. Becoming less available. Which is sad because all you want to do is help, by the sounds of it, you presumably enjoy the role & ofc love spending time with your DGC. Unfortunately your DIL for w’ever reason isn’t altogether comfortable with that. But is happy to continue to use your services, all the while letting you know of her subtle disapproval (or w’ever it is).
Ok if she sells some gifts if the place is over whelmed with toys & stuff. But there are ways of showing up in relationships & your DIL is showing you hers. Good luck x