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It looks like you followed a pretty good plan, the only thing I would recommend at this point would be to make all of your 301 redirects are 1 to 1. What I mean is, go over your 301 redirects to remove redirect chains, so you will update all of your old redirects that are pointing directly to the new https pages instead of chaining those redirects.

I think you would see an overall decrease in your ranking for your key search term.  While the function and rank of a page are important, that page's ranking for each of your websites takes into account many domain level attributes which would not be replicated on a new site that only had your calculator. You better bet would be to work on optimizing not just your calculator page, but also each website as a whole to help increase your position on that keyword.

I think you answered your own question:  "these sites have IP block so it requires an VPN to access with Japan IP"

I would avoid trying to get links from off language sites as there is a greater chance, that even if relevant, those links will be considered spam links by search engines. Now if you have multiple language tags onto your site (but no url changes) then you may be ok, but if your content is only in one language, you should try to get links from sites also in that language.

When you are using dynamic content on a page, if done properly you should have a no-information option, this is the content that will show for anyone who comes to the page but your CMS does not have any information on them.  This content (what you could consider the page's base content) is what Google will see and rate for relevance.

The big question is how long ago did the URL change take place.  I have had similar circumstances when gaining new clients who had been working with some less then ethical companies and we found that after ~6 months there is little SEO value unless there are still good, active links pointing to those locations.  If there is still some good links pointing to the original location then you should regardless, but if not and it has been longer than 6 months I personally would not.

The point of a sitemap is to tell Google what is on your site so it can index it easier. There is no way, nor any reason why you would want to put external urls onto your sitemap.