It sounds to me, at the risk of being blunt, that you've basically answered your own questions! You like the Oregon, it still works and it appears to do everything you want. You're right in saying the Montana is a brick - I use it on the bike only when I have aerobars fitted and I can use a home built mounting arrangement using a RAM mount to nestle the Montana down between the aerobars. The visibility issue I can't comment on, since I've not used an Oregon, but I can say that the Montana isn't quite as readable as my Map62, which is not a touchscreen and in my opinion, is the better for it. I don't know if the Monterra has made any meaningful progress in this regard.
The major benefit for me is the satnav option with my own Tracks/Routes, which very few conventional satnavs do (or at least, used to do when I bought the Montana) and the extra screen size, which is handy for me since I use it also on a recumbent, when it's further from my eyes and which presents a whole fresh set of problems!
The speed to relocate after having moved continents can't surely be a problem you encounter all that often? I've taken a variety of Garmins on holiday in Europe and they've all taken a few minutes or more to get a fix. I'm aware of this and allow time for it.
I'm using a variety of Garmins, accumulated over time, and if you forced me to get rid of all of them bar one, it would be the Map62 I'd keep.