Lol you just compared going to a website and changing location preference to something that requires keeping a ledger. You do you, but I'm fine this way haha.
You're missing the point, and it doesn't really need a ledger. You can just use the same password across all of them but suffix the password, like add -jpn, -usa, etc on the end. Easy to remember.
My way: you never need to leave the Switch. You can access any store you want while lying on your sofa, in bed, etc. You can buy from any of them immediately and use it on your default account/any of your accounts.
Your way: Fine once or twice, pop to the website, possibly jump through a hoop, lots of fiddling and clicking in my experience - but if you do this a few times a year or more it's a faff and you can't just browse and buy from any store at your leisure.
You can I'm pretty sure add multiple + signs into your email, and I know this is the same with the "." if you add it into your email. And if you use gmail filtering you can label all your Japanese emails, Nintendo all that into their own place by just filtering the +nintendo/jp and it works just like magic.
And yes, this is a classic trick too.
If your email address is
myname@gmail.com you can make another nintendo account but put in
m.yname@gmail.com. Gmail doesn't care, it all goes to the same place, but you can be cheeky and have multiple Nintendo accounts linked to one email.
The reason I didn't recommend this is that certain tech-savvy systems and websites cleverly dodge this and tell their account systems to ignore punctuation before @s.