I have often worked for companies in Japan, and they have just "sent me the money". So, I'd email them an invoice for €XX,000 and they'd (eventually!) wire €XX,000 to me.
(Then, obviously, and unfortunately, me or my company would pay all the usual taxes wherever I was living.)
It could be that the missing ingredient in your formula is you need some sort of company entity? (Often very simple to set up, even in Italy.) BUT many/most Japanese companies would be Troubled by such an arrangement.
Note that Italy-Japan, of course have the usual "double-taxation-avoiding" tax treaty, example
(Just search on "Italy" on the page.)
I have indeed heard that overseas "employees" unfortunately do pay about 20% tax, to Japan (although, basically, they'll "allow for that" in Italy) You can (unfortunately) easily google this anywhere, eg https://home.kpmg/xx/en/home/insights/2011/12/japan-income-tax.html
I have never heard of it "reducing" to 10%? That seems odd (unless you are moving there or something?)