Drat, the pesky problem of the gains not being all yours when you realize them!
I agree with your thinking, especially given that you don’t intend to fill your NiSA up with fresh funds next year.it will be sold at the rate in December and the rebought asap in January.
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My thinking is move it to the NISA!
Any reasons NOT to move it?
What I might offer though is to not be too eager on doing it in December / January. I would take a wait and see approach. After a possible Santa Claus rally may not be the best time to sell. (If the world is collapsing in the wake of upcoming political events then maybe it would be but that’s not my base case).
(secondarily if you do furusato nozei then selling in the new year is definitely preferable since you’d leave yourself little time at year end to do the furusato nozei donation)
Imagine if the market corrects by 10% some time next year (or currency movement creates a similar effect) and you sell then - you would realize less profit (boohoo), and thus have less tax to pay (hooray), before reinvesting into nisa at a lower price.
When to sell is the difficult part obviously, have to see what is going on in the world. But to avoid time out of the market, you could consider splitting your selling / buying into smaller chunks, so that after you get cash for the first sale you can sell the next tranche and buy back a tranche in nisa at the same time, minimizing the amount of money you have out of the market - assuming you are working with no load mutual funds, and don’t pay transaction costs.
Whether it is worth the bother to try to save on some tax comes down to you
If you say sell 2.6 million worth, realize up to 1.3 million in taxable profit, it’d make for circa 260,000 capital gains tax. But if you could sell lower and pay only 200,000 capital gains say, that’d be the difference… 60,000 yen immediately but tax protected thereafter.
Personally I will try to amuse myself in this way, for even less possible tax savings