While Mathcad Prime and Mathcad 15 are rather different and the switch from legacy Mathcad to Prime will take some adjustments, Mathcad Prime has a number of advantages and enhancements over legacy Mathcad (15 and earlier). I’ll resist the temptation to enumerate all of them, but I definitely encourage you to continue exploring Prime. Regarding your three questions:

1) Establishing formats in templates is very easy to do in Prime. You can change your default formats for text, math, document layout, etc, and save that as a template file. Then you can tell Prime to open that template as the new default every time you open Mathcad Prime
2) While Prime does not give users the option to modify the appearance of the definition operators, this is for good reason. Without unique syntax for the definition operator, it would not be immediately apparent what values are being defined and what are being evaluated. This is important because one of the major focuses of Mathcad Prime is to produce an engineering notebook that enables both users and viewers to easily and immediately understand what is happening in the calculations. That said, it is understandable that users would like some flexibility in how they display their calculations, and to that end, consideration is ongoing for including ways for users to customize the displays of certain operators in future versions of Prime.
3) Both legacy Mathcad and Mathcad Prime have excellent capabilities in unit handling and unit conversion. However, one of the major advantages of Mathcad Prime over legacy Mathcad is its ability to label variables, constants, functions, units, etc. This makes defining new units extremely easy. In Mathcad 15 and earlier, you created new units as simple variables. In Prime, you can actually designate a newly defined unit with a unit label, which will cause that unit to appear with different formatting (default is bold, dark blue letters) to alert users and viewers to the fact that it is, in fact a unit. If you or your company uses the same custom units consistently, I would suggest building a worksheet that includes the custom unit definitions in a collapsible Area, and saving that as a template and then telling Prime to open that template as your default, so you have your custom units ready to go every time. To be clear, this is a different workflow than could be done in legacy versions of Mathcad, but in Prime you should still able to take advantage of custom units in much the same way.

How does this differ from Mathcad 15? I create and use my own templates, including a modified default, and have done so since at least M11. From your comments, it would seem that I have the ability to actively manage which template is the default, but given that the default worksheet is likely to remain as such for long periods time, I'm not sure that it adds greatly to the capability.



How does this differ from the use of Math Styles in "legacy" Mathcad? Again, I've used those for years to distinguish between scalars, strings, arrays, function arguments, local variables and units. By renaming the styles to have a unique first character, they're reasonably easy to apply with the key sequence Alt-0,Enter,<style character>,Enter, as well as from the drop down menu. Styles could definitely have been better supported from a user perspective, but they were always there.