Eventually you won’t have the credits to go around. Really there is no reason to upgrade one card over another. They are functionally interchangeable, I just tend to upgrade cards I use.
I will say that always do the level 1 upgrade. It is the most efficient, credit wise. 25 credits for 1 level, as opposed to 100 for 2 CL. Personally once I do the level 1 upgrade for all cards I try to focus all the rest on maxing a single card. Not always possible but there you go. There is a mechanism when the card gets to rank 10 (infinite, last upgrade) you can do an infinite split and start a new copy of the card at level 1, but it has a holographic background. This is a good way to keep using boosters for cards once you max it, as well as a nice reward. You won’t get there for a while though (usually just before getting to pool 3).
But where you are, just always upgrade everything is the right call.