Obviously, you need to keep your growing hair trimmed around the sides so that it does not get to be visibly longer than the system hair on top. The shorter your haircut is, the more often you will need to have it trimmed and maintained. Longer hairstyles afford some advantage of needing to be cut less frequently. So there's that.
But it might help to clear up what is meant by "blending". when you talk about cutting in hair systems, "blending" can refer to removal of density or weight from the system to match your growing hair more closely than it does out of the box, especially around the perimeter. As Noah said, you only need to do this once.
Whereas when you say "blending" I assume you refer to a gradation in the length of your growing hair, e.g. a haircut that is tapered around the sides rather than cut to a uniform length? It would probably help if you could post some pictures of the kind of haircut you want to achieve, otherwise we're all sort of clutching at straws here tbh. I'm under some impression that you've got a pretty short haircut currently.
Ideally you would be confident trimming your own hair when you start wearing hair systems, though it isn't necessary. My suggestion would probably be to put this time toward developing that skill, it's not too hard but you probably won't get it perfect the first few times. If you still have a lot of hair you can also experiment with concealers like toppik and derrmmatch (which can also be used to mask flaws in DIY haircuts gone wrong).