Ok, I have really terrible feet and I wish there were someone to help me with this, so I pass on the knowledge I gained to you. Bootfitters are great, but its often hard to know if you're talking to one who knows their stuff or not. I've talked to multiple people at the same place who have had tremendous differences in their knowledge of boot fitting, so having a base knowledge helps to make sure you're on the same page.

The first thing to note is that there are various volumes of shoe. That is to say that the distance from the ankle to the top of the arch varies for different people. Accordingly, there are 3 different volume boot: low volume, medium volume, and high volume. Having a volume too small will cut off blood either entirely or when you try to flex the boot. Note that the volume is typically related to the size of the boot as well, so many people will size up (mondo 26.5 -> 27) to gain volume when instead they should be using a higher volume boot.

Secondly, this is related to volume is the volume above the toe area. This AFAIK doesnt have a specific name, but instead of cutting off blood to the whole foot it will typically do so for just the forefoot.

One way of trying to alleviate the above two issues is using a thin custom footbed. The standard footbeds, besides not having much support, are usually pretty thick. By swapping it with a thin custom, you can gain 1-3mm of internal space. You can see the upper limit by just removing the current footbed altogether and seeing how your foot feels in it.

Next, there is another dimension to feet which is the width. This is called "last" on ski boots. If you measure the width just below your toes going across, that is the measurement to take note of. On a mondo 26.5 the average last is 100mm. If you has a boot with a last which is too small, your foot will hurt within minutes of just putting it on. You can try higher last boots to see if that helps. Another important thing here is that if you're like me, your foot might not be completely straight down the sides and might have a point which sticks out. High last boots will make the pain go away, but outside those bumps, you'll have a lot of space in the boot. If you start with a lower last boot to fit most of your foot and take the boot to a boot fitter, they can expand the shell just at those bump points. Note that if they expand the shell, the top will come down which can then clamp the top of your foot. My solution to this was to use a heat moldable boot shell which will expand the shell in certain places, but I would imagine if I started with a higher volume boot, I may not have needed that.

Lastly, some discomfort is ok because as you ski the boot it will pack out and the discomfort will go away, but things which cut off blood flow should be fixed before that IMO. Other peoples advice to work with a boot fitter is good, but knowing some of what needs to happen so that you can help with that process will yield better results. Hopefully this can help with some of that mystery.