Your experience as a native speaker is completely different from learning the language from scratch as an adult, to the point that it's almost irrelevant. Writing Russuan is not that difficult, it's just the only part that you had to actually do any work to learn
Poles will hate me saying this, but I've always really struggled with their orthography, even though I am used to the Roman alphabet. I can see what is going on in Belarusian, Russian and Ukrainian, maybe even Czech to some extent. Polish is bizarre. Szcz is one letter in Cyrillic. I'm still baffled by l with the line through it.
> Writing Russuan is not that difficult
Never thought the difference mastering writing can be so significant. Just like to add what I understand regarding this. It's rather about not making any mistake writing by hand ca. 1-2 DIN A4 pages while someone reads a text (slow enough). I can't remember exactly but making only one (or two) mistake(s) and it is not anymore excellent (just good). Making 4-7 mistakes and it is not good (just sufficient). Making few more and it is bad which means failed. It's a long text with a very short path to fail.
Ukrainian is less difficult to write. There are claims that standardization/reform of Russian made it more artificial (far from natural people language) with overtaking too many words from Latin languages. When I read / listen to Belorussian I think they have even more luck with matching pronunciation/writing than Ukrainian. Which suggests this language is even closer to the common roots old language. (I'm not a linguist.)