@John-S-Parry I totally understand and I mostly agree. I've used both the OSM and HERE maps extensively in MRA.
However I do believe that the "lowest common denominator" between MRA and Garmin are the HERE maps, and my best chance of seeing my exact route that I planned in MRA on my Garmin device is using the HERE map in MRA. I think that this belief is echoed by many of the top MRA experts on this forum (which I AM NOT!).
Is it perfect using HERE maps on both MRA and Garmin? NO, absolutely not!
Is it better than using the OSM map in MRA, then saving the route as a .gpx file and importing it into a Garmin which uses the HERE maps as its base maps and not the OSM maps? Based on my experience.... YES.
I really like the OSM maps.... don't get me wrong. My goal when I create a route in MRA and transfer it to my Garmin is to follow the route EXACTLY as I planned it in MRA.
In MY experience, MY very best chance of accomplishing that goal is to:
Plan the route in the online version of MRA using the HERE map, and put via points where I definitely want to stop or at least pass by (petrol/gas stations, overlooks, monuments, restaurants, or whatever)
Put enough shaping points in MRA to shape the route exactly like I want to ride it. This is where MRA excels and makes creating exactly the route that I want easier than any other application that I have ever used.
Save the route using the .gpx 1.2 format
Import the .gpx 1.2 route into the Garmin and place it into a collection along with the track so that I can see both of them simultaneously (this is for the XT2 and XT3.... previous models don't use collections so there are other methods for showing routes and tracks simultaneously)
Avoid skipping via points as much as possible, and if I must then I just restart the route and enter at the closest entry point.
I think that we all realize that EVERYTHING we do between MRA and GARMIN involves compromises. If there is a "perfect" solution I most certainly don't know what it is, nor have I ever heard of anyone who does.
I believe that the best we can do is weigh all of the variables and do what we think is best for our application. There are really no right or wrong definitive answers!
The key is understanding the options and limitations so that you can make an intelligent choice, and I'm very thankful for all of the expertise and thoughtful input everyone has provided here. I have learned a lot!