Question about international taxation and accumulating ETFs — looking for guidance

Hi everyone,

I’m fairly new to international investing and I’m trying to better understand how taxation works when moving between countries.

For some context: I’ve already maxed out my NISA each year, but I’m also interested in investing through Interactive Brokers to access a wider range of products. Where I get really confused is about the taxation of capital gains accumulated abroad.

Here’s a scenario I’m thinking about:

• I live in Japan for 10 years and invest in an accumulating ETF.
• I never withdraw the money during these 10 years, everything stays invested in the ETF.
• After those 10 years, I return to my home country (Germany).
• Two years after returning, I decide to sell the ETF and withdraw the money.

My questions:

• Would the gains realized during the 2 years in my home country be taxable there?
• What about the capital gains accumulated during the 10 years abroad, could they be taxed by my home country or by Japan, or would they legally not be taxable anywhere?
• Would this scenario count as legal tax optimization, or am I missing an important rule?
• Are there specific nuances related to international tax treaties, foreign accounts, or accumulating ETFs that I should be aware of?

I know this is a bit theoretical and simplified, but I mainly want to understand the general principle and see if I’m overlooking something important before investing seriously internationally.

Any insights, experiences, or reliable resources would be super helpful, thanks in advance!