I don't really think there is any way to avoid "bad purchases" or "buyer's remorse." You can only be completely sure of your purchases and decisions once you decide what REALLY matters to you. And, unfortunately, the only way (I know) of accomplishing that is through trial and error.
Anyone who has spent any amount of time around clothes will be the first to tell you there is no such thing as the perfect fit, or the perfect garment, or the perfect "outfit." When you get down to nuts and bolts there are always areas to improve upon. So, to avoid chasing the dragon, I suggest figuring out what matters most.
If it's "fit," then focus on what aspects of fit immediately stand out to you -- shoulder expression, drape, back, balance, buttoning point, etc. Once you figure out where your eye looks first, you can go from there, with each iteration (rather it be alterations to RTW or MTM or bespoke) improving upon an area that you truly care about. You start with what matters most to you and then tweak from there.
Others will say fabric is paramount to fit. If you're in that camp then you need to make sure that is the first question that crosses your mind. Are you a "hand-finishing" fetishist? If so, then best to know which makers/tailors are going to show you the shoulder balls. And so on, and so on.
The point is that there will always be a bunch of high-level guidelines or bits of advice. However, especially for someone starting out, not understanding why these bits are advisable makes them more or less useless. And beyond that they actually open the door to wasting more money or time because they're going off of the advice of someone else's endeavors. It leads to people obsessing over every detail of fit only to realize their suit is made of sandpaper or spending years hunting down the perfect cashmere blend only to realize a readily available one would have been all they really needed.
While I like the spirit of the thread, I think the takeaway should be, "Take a more studied approach to dressing." Rather than "5 Weird Tricks to get 6-Pack Abs."