Try to take everyone's opinions for what it's worth. I know you are asking a lot of feedback but at the end of the day, YOU need to love your ring. Whatever makes you happiest in person (color, clarity, branded or unbranded) will be the right ring for you!
I honestly cannot think of one person that doesn't know that unbranded anything is always cheaper than branded. So obviously, those that chose to purchase from Tiffany knew from the beginning that the prices are going to be a bit higher. And those that didn't purchase from Tiffany will usually always tell you that no one knows where the ring is from, you aren't wearing the blue box...
Tiffany sells beautiful jewelry. Their settings are TDF and I think for certain designs, it's best to just get the ring from Tiffany than getting an "almost looks like."
Example, you love the Legacy ring. Well, it will be hard for a custom jeweler to get the proportions JUST right based on your individual stone and the 2 pictures provided on their website. The tiny pave diamonds are full cut (all 58 facets instead of 19), so if the jeweler did not do that you may not get the same quality of diamonds in the setting or that may affect your price quote on a custom ring. The Legacy has a different cushion than what most brokers sell as a cushion. It has less of a crushed ice/vintage glow and more of being a softer Asscher. If you only like the setting and want a different center diamond cut then custom from a local jeweler is definitely the way to go.
Try pricing out like for like: the setting, a diamond with the same cut (most people will quote you a generic cushion from an online vendor... which doesn't mean that it will have the same look at all), cost of getting a certified/ethical diamond. If it's still significantly cheaper, I would at least consider it. If you don't think the difference is that big of a deal/ close enough than I would get the ring from Tiffany's.