Are you doing the special missions (golden diamonds on the map) especially after getting a new certification? If not, there’s your problem. For one, they pay a lot more. Once you get a certification, a special mission will pop up just for it. Completing that will not only pay well, but that’s what also unlocks that type of mission worldwide.
It took me a while to figure that out. That only happened because I got bored with the basic flightseeing missions and nothing else available, even after having more certs and specializations available. I saw those gold diamond icons and noticed I could do some, and now I’m advancing quickly. I’ve not felt pressured in the least to spend extra money on fast tracking career mode, nor have I seen a way to add credits to my career balance. I’m pretty sure the sim credit purchase you are seeing is for making marketplace purchases. That’s how it was in 2020. You didn’t buy the item outright. You bought a certain amount of sim credits, and then those would be used to make the item purchase.