I had the Edge 800, 810, 820, 830 and now the 840 solar. I'm a garget freak!
The 830 was a huge leap up from the 820 due to the Trailforks integration.
The 830 to 840 was a waste of time really, and the Solar function is practically pointless. I could get the same extra battery gained by solar on a 3 hour ride as plugging it into the wall for about 2 mins. Apart from the buttons on the side, there's no advantage to it (for me anyway).
Anyway - like you, I'm in Scotland and sometimes travel to mountains or trails I've never been to before.
Before travelling, I look on Trailforks for routes or trails that I want to try. I've done a few pre-defined ones that others have created, but Trailforks also allows me to "draw" out my own ride, by basically tracing the path I want to take with the mouse. I can then save it to Trailforks, and download it on my Garmin using the integration. I read reviews of trails, and try and make a route that incorporates all the ones I want.
Yes, the Trailforks on the Garmin is somewhat limited compared to the phone app... but what it does allow is turn by turn navigation of the route I've laid out. I've explored miles of the Glen Tanar estate near Aboyne by making up random routes and following the Garmin. It means I don't need to stop and take my phone out to check whether I'm turning left / right at the crossroads. It flows really well and gives advanced warnings of where I'm going, and also shows the climbs as I approach them and how long / steep they are (that might be a new feature in the 840?).
The other cool thing about Trailforks on the Garmin is that you can be riding along a landrover traick / fire road, and notice a trail off to the left / right on the map that you would never have seen otherwise. I found quite a lot of really good 'hidden' trails this way. Again, unless your cycling with your smartphone in your hand or on your bars, you wouldn't get that info.
I've been in the Garmin ecosystem for a long time now with Golf "Approach" devices and bike "Edge" devices... which is why I'd struggle to move to something else.