> If you write on paper, you need to write out each character by hand.
Exactly like English or French. You also need to remember the correct sequence of letters (see my remark below).
> In contrast, if you write on a tablet or other device, the input system will sort of "autocomplete" for you.
Exactly like English or French. I didn't write half of the letters used to type this comment.
I think you are searching for Japanese exceptionalism where it probably doesn't apply. Remembering the correct kanji, and graphy of said kanji is pretty much the same as remembering the (wild) orthography of French or English words. I'm searching quite often if a letter is a 'a' or 'e' (doesn't help that cognates between French and English often swap those), the conjugation of a verb or if a letter is doubled or not (again often not the same for related words, for example ennemi vs enemy).
Your perception of the difficulty in-between writing systems is twisted by the decades of education you got in the said system vs the few years of learning another system with a second language. When I see some soup of letters in Japan, it's clear that the question is not just about one system being more simple than the other.