Thanks man. I have a 2019 Titanium 2.0. Do you need to physically take the vehicle somewhere for tuning? 

Good callout on the stop/start accumulator. The solenoid on mine wasn't loose, but the seals on the solenoid pooped the bed and puked fluid in my driveway. Easy to replace yourself. 

For a squishy ride try 235/70-17s or 245/65-17s depending on wheel width. A wider tire on a narrower rim will give a softer ride than a narrow tire on a wide rim within reason!   But, according to the chart below, the wheels are only 7.5 inches wide or unfit for your needs.   https://www.wheel-size.com/size/ford/mustang/2018/   The best you might do is 18 inch wheels.   https://www.wheel-size.com/size/ford/edge/2018/#generation-ii-facelift-2018-2024   Check local wrecking yards too.    

You can do the tranny at the same time! Pop 2 plugs, drain, add the fluid (Ford has built-in a handy funnel holder), drink your favorite waiting beverage for the fluid to reach 180 F about 10 minutes and install the site plug!

I've already checked the wheel offset/backspacing, lug spacing, and rim widths between the Edge and Mustang wheels - compatible.   The rotor diameters are the same, but the calipers are different, which may be the real issue rather than the rotor diameter.  As a side note, the SE version of the 2012 Edge came with 17" wheels.  This limited version was stock with 18", but by the window sticker for the vehicle, it had the optional 20".   The tires are Cooper Evolution 245/50-R20, and I've tried running them at 36 psi and down to 30 psi, but it doesn't make much difference, so I'm considering switching to wheels to install tires with a taller profile.  The vehicle will lose a little steering and cornering feel, but this doesn't get pushed by my wife as I would do.   If I didn't have to drive 90 minutes to see how they fit, I would have already tried fitting them.  This is why I'm asking.  I'm skeptical how anything in the 50 profile is going to soften up as much as a 65 profile.  But if someone has found one, I'd love to hear about it.

I feel pretty confident doing my first PTU and RDU service. I'll probably take it to my dealer for the transmission service.  More so, just concerned that they actually do it. And do it right!   Being on this Forum gives me so much valuable information. Including a lot more confidence in doing some of my own maintenance. Thanks to everyone!  

This is a whole lot easier than changing fluid on a C4. Then bumping the engine over to  find the drain plug and draining the Torque Converter. Not to mention somewhere between 10 to 13 Quarts. Did I forget the refill procedure? To lazy to type it out!  These things are easy to DIY just HOT!

So you have to go to the dealer for service

You have to ask yourself, " Why do you need these ? "   They claim no additional NVH to the cabin, but where are the videos to confirm this? They also do not say how this will affect ride quality, if it even matters to you.   What are these things made of and what is their life expectancy?  These may be nothing but a plastic insert and wear out quickly. For $300, I would want to know more

What exact brand tire doo you currently have and what air pressure are you running in the tires?   Before you go making this swap, you are going to have to do a little homework.   Determine the wheel offset of your 20" wheels. Determine the wheel offset of these 17" wheels. This will tell you the amount of backspacing you have / will have.   What size rotors are on the 2018 Mustang? What size rotors are on your Edge? If they are not the same, you are going to have a problem.   You also have to look at the inside design of the 2018 Mustang wheels and your current wheels. This is another potential clearance problem.   Quite honestly, I would not do this.  Too many potential issues.   If the real reason for doing this is a ride quality issue, the easiest way to fix this is: 1.  You could lower your tire pressure a small amount 2.  You could replace your tires with ones that have a softer sidewall.   The easiest way to find out is pull one of your wheels, pull one of the Mustang wheels and install the Mustang wheel on your Edge and properly bolt the Mustang wheel down. Now spin it by hand and see if there are any issues.