What would really be useful to them (and me) is an app focused on how people really talk, that teaches actually commonly used phrases as well as grammar and vocabulary.
Apps focused on reading often produce people who can read but can't have a conversation. This is one of the main criticisms of the way English is taught in Japan by the way.
PSA: That's not an actual barrier. It's a borderline useless thing that some significant subset of self-learners seem to do, for reasons I cannot explain.
There is no such thing as "learning the characters". You need to learn words and sentences. You can certainly categorize the learning by character (this is how it's done in Japanese language schools), but that's not necessary (or necessarily advisable), and going from that to "I must first learn the characters" is pathological. People get stuck on this because they spend weeks/months/years doing rote memorization, and make no progress toward understanding the actual language.
Eventually you get to a place where you can start to infer word meanings from kanji, but that's a lot like using latin roots to infer the meanings of unknown words in English. It's a nice-to-have, not an essential tool, and you'll get there anyway if you simply study words.
I agree with you though, you really benfit from the dense web of association you get from actually reading and hearing words in sentences. Much quicker path to practical fluency in my opinion, and you get the benefit of learning words in contexts you will actually use.
I made the kanji decks in my other app Nihongo (https://nihongo-app.com) based on this philosophy. Basically, they teach you (1) the kun-yomi word(s) for each character, (2) the 1-3 most common words for each on-yomi, and (3) to write the kanji.
Don't try to learn the readings in isolation, learn them in the context of the words where they appear. And by learning a small set of the most common words using the character (especially if it has a standalone kun-yomi word), you'll internalize the meaning of the kanji itself.
For most people kanji is better learned with vocabulary.
You definitely need to learn hiragana first, and not use romaji at all, but that can be done in 2 weeks.
Just having to learn those first will stop a large percentage of people who would otherwise be able to learn to talk reasonably well.