Beyond the obvious "lurk moar" comment, which, to be honest, is always the answer since no one on the MC side of things really knows what they pretend to know (pardon a few members), I think what you need to realize is that you are about to undergo a shift in thinking.
Pardon my sweeping generalizations or hackneyed ignorance here, but the mentality of SW&D and MC is vastly different. That does not mean one is better or one is proper. They are just different.
The issues that you felt made you nervous regarding bespoke are very typical. Even those with the most money to spend do not walk into a tailor's shop and come out immediately with exactly what they want. Instead, the bespoke (or even MTM) process is one of trial and error. You decide what you like, what works best for you, the details that keep things interesting, etc. Your passion and the hunt for the perfect fit keep you running like a junky. And all the way there is your dealer tailor to be your guide and keep you in line. You will go through a ton of commissions before you settle on something you really like. And then, as soon as you have the default/conservative arena covered, you strike out into goofy shit to keep from being bored.
To me, this is the biggest difference between SW&D and MC. In MC the ideal (for better or for worse) is the bespoke process. Having garments made for you, to your exact specifications, gives those of us who slave over the details a great deal of satisfaction. While there are a number of custom avenues in the SW&D world (or just street wear scene in general), there is no ultimate step that involves complete customization of everything. Instead there is a lot more prepackaged style and highly designed brands/items that require a great deal of understanding. Yet, at the end of the day, they are still not your creations.
Before you jump into bespoke you need to start realizing and appreciating the finer details of tailored items. Things like pocket configurations, stitching, hand finishing, button stance, fabric selections, lapel details, etc, etc are all things you should start paying attention to. I guarantee, aside from overall fit, if you compare the details of the Dior piece you mentioned with any of the WAYW greatest hits, you'll see a number of things pop out. It's not easy to get this right away, and you'll see that you miss a ton of shit that the MC nerd crowd will be quick to jump on. But again, it's a process. You need to first see the details. Then understand why they are there (or why they shouldn't be there). And then appreciate why you think they are cool or terrible.
From this basic point you can move into more complex areas like the formality of certain fabrics, occasions for certain styles, texture and combinations, etc etc. However, attempting to create an eye for the detail before jumping in feet first will save you a shit ton of time, money and frustration. Especially when you are coming from a completely different mindset.
Once you have an understanding of what goes into dressing "correctly" (I put this in quotations because what is correct is completely subjective, learned and earned over time, and in no way dictated by what people post on this forum), then you can begin to look at things on a more personal level. Get an idea of your proportions. See what you think could be improved by structure, what should be left soft, etc. A tailor is your greatest ally here, but you'll know you have a good idea what you're getting into if you can look in the mirror and spot obvious flaws in OTR garments. Again, this only comes from doing your due diligence.
It's funny that the title of your thread alludes to "when" you make the shift in thinking. In reality, it should be "how" you make the shift in your thinking. As you observe and learn you'll soon realize that there is no on and off switch. You'll carry over ideas that you like from SW&D. You'll create new ideas based on what you see here. The combination of these will make for your style. And that's what's at the heart of it. Being here isn't about thinking in terms of MC or in terms of SW&D. It's about thinking in terms of your style.