Oh, that I can help you with.
Here’s the advice of the century if you move a lot (and even if you don’t, actually): hire a competent tax consultant. He’ll take care of everything: tax returns in the right countries at the right time, and declaration of bank accounts and personal wealth in the right countries.
It’s really worth it, because it’s all very complicated: what you declare and what you can deduce in each country depends on the countries’ mutual tax agreements with one another. If you want to pay exactly what you should and not a cent more, you really need an international tax specialist who keeps on top of things.
Also, taxation is done different differently in different countries, and it can be a bit of a challenge to learn what you have to do to avoid problems. The tax advisor knows.
Other benefit: if they fuck up, they pay. When your tax return is done by a specialized company, the taxman tends to think twice before auditing you. And I can always sue them if I supplied the right paperwork and they failed to declare my taxes properly. That hasn’t happened to me yet though.
And finally, it’s incredibly boring and pointless work to do your tax return - even if you only file in one country and you’re anything more complicated than a simple employee with a single source of income. Personally, I decided a long time ago that I only had one life with a finite number of hours to spend on this beautiful planet, and I ain’t got no time for that sort of shenanigans.
I pay my accountant 400 euros a year and he files my tax returns in the 7 countries I’ve lived in. He just sends me a mail every once in a while asking me “Don’t forget to send me this or that paper from your employer before this or that date” or “Can you ask this or that bank to send you a statement, and then send it to me before this or that date”. That’s all I have to do. It’s money EXTREMELY well spent for the purpose of never have to deal with that shit.
Here’s a little secret: I’ve never filled a tax return form in my entire life. I only know what they look like because the accountant sends me a copy for my archive