Sync Sideloaded Kobo Annotations With An iOS Reader?

[FrozenTrout]

Hey, so there's lots of information on dealing with Kobo annotations in the forum, but it's hard to sort through it all to find what I'm looking for.

I recently converted my dad from over a decade of Kindle use to Kobos. He's loving it so far, the firmware is so much nicer to use. However, there's one particular problem he's run into.

He likes to make and refer back to annotations while he reads. He did a combination of it on his Kindle and the Kindle iOS app on his iPad (the annotation UI is much easier/faster there), and it would sync between both.

Now, that kinda breaks down. There's a Kobo iOS app, but he has mostly sideloaded books, which the iOS app doesn't read or sync.

I'm wondering, is there any way to sync annotations both ways between a Kobo device and an iOS ebook app? I'm guessing there's no way to do so from the Kobo wirelessly, and would require syncing with Calibre, but I don't know what, if any, iOS apps could connect with it.

Alternatively, if there's no way to do so with the built-in Kobo reader, is there any way to do so with Plato or KOReader? Using the default would be preferable, but if there's no other option I can at least offer it to him as a possibility.

Thanks!