It seems the main thing is to recognize the suit itself is only a suit--either way.
I do think that as a general rule it's bad, maybe even super bad, to buy something specifically "so I can fit in it later" ( because of the issues of "how" something fits that goes beyond mere size. That is not the same as keeping something you fit into last year or buying something that you intended to fit now but find it doesn't quite fit after all and thinking about the cost: benefits of returns. And it doesn't have to mean the suit is a "gateway suit" to a habit of buying those too-small sizes.
So the point is more about how you feel--do you feel fine and pragmatic--suit might fit so will see, if not will pack up and send back; or do you feel anxious and super-invested in this one suit and determined to make it work and start buying things to go with suit-that-does-not-yet fit so you'll be distracted from your main goal of health and finding fun new clothes all along the way?
In this situation I might project my feelings onto you, but that might not be what you will experience. But all of the points expressed might help you see shat resonates the most.