As someone whose mother tongue is German (I’m from Austria),
I wish you good luck with learning it!
It is an easy language; the different tenses will be a bit tricky. German has more tenses, and Japanese is simpler and has fewer. German also has pronouns — which don’t exist in Japanese and English in the same way — it’s just “that, sore, are, kore” (as far as I know in Japanese).
In German, it is “die, der, das” — and here you also have to adapt them based on the gender and case of the noun, which don’t exist at all in English or Japanese. For example, in English, it’s just “the woman, the man, the dish” — no gender change, no case change. In Japanese, it’s also just sore, kore, are (or sono, kono, ano when used before a noun), and they never change — no matter whether you’re talking about a man, a woman, or a thing. Oh! And the capitalization! It doesn’t even exist in Japanese, and in English it is way simpler than in German (I hate it).
But I am sure you will manage and get the hang of it!
I think that your motivation, and why you want to learn a language, always plays a huge part in how long you will study it. And as you said — Germany is nearer to Czechia, so it is easy for you to visit, you will find people you can practice with and hear the language, and it’s not as far away as Japan. (Just my thoughts.)
But again, you don’t even have to explain yourself to anyone — it’s your choice.
And I have to agree with you on the SRS method and immersion! My schools never mentioned such a thing or learning types in general! I also studied in Germany for a bit — it wasn’t mentioned there either, even though I visited a language school. Nor did we ever learn that way — why??? It is sooo much easier. I would have wished I knew about it earlier.
Maybe you will find a similar website as Bunpro for German which offers such functions. Good luck!