It really is just a claim, although I am working hard on proving my claim (if it’s even possible). Yes I also agree that onomatopoeia very strongly convey the base meanings of kana, although it’s very difficult to describe in English, due to Japanese and English having different language deixis. A book I usually recommend is ‘Learn Japanese from sound symbolism’, by Masamichi Watanabe. Unfortunately the English translation of this book is pretty average, but it is by far better than nothing. The Japanese version is far better.
The only problem with this book is that Masamichi understands Japanese, but doesn’t understand English, so he doesn’t ‘bridge the gap’ very well in understanding. What I mean by this is, imagine if you lived in a house your whole life (never left the house), and tried to explain to someone what a house looks like. Masamichi describes Japanese from within Japanese ‘the house’, not realizing that foreigners can only see the ‘outside of the house’, not the inside.
I am very… very slowly putting together a theory for the kana though. But it is taking a ridiculous amount of time and research into the history of the language… And I may well be wrong anyway, I just enjoy researching
As for the origin, I am not sure if you mean where I got the idea from, or where I think the actual language origin is, so i’ll do both.
Personal origin - honestly I have no idea, the more I studied Japanese, the more I could ‘feel’ the meaning for each kana, so I wondered if there was a reason for this… Or if I was just crazy
Language origin - My best guess based on my research is Kukai, he was said to be the man that invented kana, reinvented the way Japanese uses kanji, and was deeply buddhist. I believe he may have fundamentally influenced the way modern Japanese works. I could talk for hours about Kukai, but it’s all just speculation based on personal research unfortunately.