I tried politely asking Apple for "option 5" above, but they were quite clear that the only option they could offer me here (Japan) would be to let me trade in my current machine for a new one as a used laptop (not as a replacement). Happy for those who got them to listen, no luck for me I guess.
As for the other options:
1) A viable solution, a tradeoff where you'd spend more and give up on mobility but end up with a card much more powerful than whatever can fit into a laptop.
2) Definitely not worth it, imho. Specced-out MBPs have a terrible resale value drop compared to base models, I have an i9/32GB/1TB/560X and the money hit would be much larger than the cost/value of the Vega 20.
3) Depending on your use case, this might or might not be the best solution: do you
really need the upgraded GPU? Or is it more a matter of being pissed off at not having the latest and greatest anymore after just a few months? (nothing wrong if the latter is the case, it's only human to feel that way!)
4) I was mildly miffed as well, but obviously this is not a solution
I guess my final solution, sometime in the next few months, will be option 1. I'm trying to figure out whether I should go for one of the current Vega options or, since I'm not in a hurry, wait and see what AMD will offer in 2019.