From the quick googling I did at work, it seems that there are different types of starches that digest at different rates. Whole grain cereals are in the slower-to-digest category *and might not get digested fully.
I personally suspect that the process of making oat milk - blending and straining the oats - makes them easier to digest and probably has an impact on GI. So it's probably a wash.
Taking on one extra dog for a few days seems like the easiest, most chill way to make a little money on the side if you are careful about which dogs you take. Those people who sit 4+ dogs at a time are insane but must pull in a good bit of money.