I haven't replaced my MBP, but have used the iPad Pro quite extensively while away and found it very workable. Upload speeds obviously affect it in the field, but I found pretty good results with using the mobile app for processing (always tweak when back of course!)
I love the total...
I believe you need to be on 10.14.5 or above to get the full benefits of GPU options. However, I believe it's a limitation of the Graphics card (I have exactly the same set-up but with Mojave)
Photos should look the same between 8.3 and 8.4 (assuming no changes to the photo or PV or course!)
If it's a large number of photos, personally I'd move in Finder / Explorer and re-link after in case there's any computer issues during the move. Within LR is fine and correct for smaller numbers. Check this Blog post that explains the two methods...
Auto on photos already processed will treat the Basic panel like it would from scratch, but won't touch any other panels you may have altered (such as HSL)
It'll also base its calculations on the cropped area should the image be cropped.
Link to the DNG Convertor if that's the direction you decide to take:
You can update your Lightroom to 6.14 and therefore use the TG-5 of course (as above)