I have a colleague from South Africa who came by his Portuguese by the same path. The world is a small place!
Since you're discovering the world of declensions, and about to do serious battle with them, let me point you to a seriously cool insight, with a link (and a caveat).
When learning declensions, we treat the der- and the ein- paths separately. And, initially, because of the exceptions in the nominative for masculine and neuter ein- words (and accusative for neuter), it seems that this is a reasonable approach. Wrong!
It turns out that the declensions for der- and ein- words are exactly the same across the entire chart of endings for both articles and adjectives! With only 3 exceptions for ein- words (masc nom, neuter nom, neuter acc).
This applies across the board for all the der-words (dieser, mancher, solcher, etc) and all the ein-words (kein and all possessives).
This was an absolutely huge revelation to me, and it was like opening a window into a whole new world.
It's only because the three exceptions are so big and so up-front that we think there are two worlds to conquer.
And then, if you take that whole cluster of “indefinite determiners” (andere, einige, mehrere, etc) and re-classify them as “indefinite adjectives,” then even the chart for unpreceded adjectives becomes breathtakingly simple.
Here's the link. Pay attention to the chart. I find the explanations confusing, and I think the term “determiners” needs to be much more clearly explained (they are der- words, ein-words, and “indefinite determiners”, with the indefinite determiners only coming into play for unpreceded adjectives). But if you can read past that to the essence of what's in the chart, the whole world of declensions becomes actually quite simple.
Best of luck!