I work in bespoke tailoring. Did a suit for a bodybuilder once, and it was a total nightmare because he would change so much between fittings

My advice - you can take in shoulders up to 1", however, it is an expensive alteration to have done and isn't great for the look of the suit. You can let bicep out up to half an inch, and you can let the chest out ONLY at the back, so if your pectorals get bigger then you're a little screwed

Wait til you're stable and then get a suit with soft shoulder construction, eg. Neapolitan or Italian cut, and get the shoulders cut quite close in. This works better on rounded, muscular shoulders than English cuts with bigger shoulder pads, as with this style, to get a smooth line on muscular shoulders, you have to make the shoulder large and there is not much room for error.

Also, make sure you get a floating, not fused canvas. Try and get the tailor to get the chest canvas on the bias rather than on the straight, it will make the suit more flexible

I would say maybe just try doing 5/3/1 2 days a week for a few weeks and get a body tape measure. Measure yourself daily for a few weeks, note changes or lac of, then tailor the workouts from there. If you notice you're growing then you could try adding some aerobic training to keep about the same size. Once you seem to stable out, then go get yourself that suit.

I completely agree. You can still fluctuate enough now to where the tailored suit may not fit like it did the day you came home with it.

Also, what's your diet?

How often will you wear it? Try it on a couple days ahead of time. If it's too small, cut back on your carbs and sodium for a couple days.