Korean could serve as a point of comparison, as modern Korean is written mostly without Chinese characters (you could mix them in if you really want to, but approximately nobody bothers), but it does have spaces between words. And yes, when you have a highly inflecting language, "where do the spaces go" is a very nontrivial problem.
It's a bit easier for Korean as it has far more available syllables (so fewer homonyms), but you could easily imagine these languages making different tradeoffs and ending up in different places.
A lot of material for younger readers uses similar spacing to Korean, like the Pokemon games. It works well enough when you know the vocabulary being used is limited, but even there it gets frustrating.
When you're reading, you're trying to identify a substring and map it back to the unit of meaning, which is effectively the kanji. That works seamlessly with Hangul, because it mostly blocks (as syllables) the same way as kanji, and, as you mentioned, the extra vowels and silent consonants provide more disambiguation. With kana, the length you have to scan to read, and then guess at (using only context to disambiguate), a root word, is a lot more arbitrary.
Mechanically kana is just less compressed, less aligned, and makes it harder to visually identify/scan for units of meaning. Obviously Korean shows that you can make different tradeoffs, but for Japanese losing the kanji would probably necessitate reworking Kana as well - and unless the pronunciation changed, it would probably need to be in a way that represents the pitch accent, which is a modifier on the whole syllable, rather than just a few extra consonants or vowels.
Agreed, I think that's a close analogy and it's wild to me how recently Korean dropped kanji. But I imagine having a lot more vowel sounds made it a lot easier. I've wondered whether the two interacted - e.g. if there were word pairs that had previously used the same vowel sounds, but changed after the switch to Hangul to be distinguishable. Do you know by any chance?