@KM I just found a vendor of OnePlus3 on eBay who ships to Japan from Hong Kong!
He actually sells both OnePlus 3 and 3T. If I look at the specs, the differences do not seem to relate to the graphics. 3T offers more LTE cover, from the little I understand. Selfie camera resolution, battery life and device color are not a priority.
https://www.gsmarena.com/compare.php3?idPhone1=7995&idPhone2=8416
Would you agree? Do you use yours fine in Europe, do not you?
Paranoid Android should work on both phones. I have the doubt which one to try.
By the way, once I install the custom ROM, if it does not work for my eyes and I want to resell, do I have any chance to factory reset?
If you have any recommendation, I greatly appreciate it, since you are way more knowledgeable. Do not worry, I won't blame another wrong purchase on you :-)