Unfortunately I am aware of this issue. I'm in touch with my local tax lawyer and I was advised not to put my games on other storefronts because of different legal basis and payout systems - Steam pays each month, itch.io whenever I ask, which kinda messes up accounting.
But as with everything, things might change, so I might be releasing games on itch.io too, in the future. Or Steam changes their system to comply with German laws (or maybe German laws will change).