To me, there’s a lot of information being conveyed in the ond here, where in English and I guess in Welsh if you wanted to, you could or would insert more words. To me ond is more like you’d here in expressions like dim ond - for only and in English you might say when or while only, but here ond is sufficient to say all of that on it’s own, if you want to. That’s my “not really qualified to really comment” opinion anyway.
I’ve just been googling and it’s very common to see a hithau ond followed by age and ac yntau ond followed by age etc. It just seems a common way to express this sort of thing - it’s verging on idiomatic to me, but I’m sure a grammar expert could disect it if they wanted to.