Is there an independent, cross-device cloud sync platform for ebooks?

I used the Kindle ecosystem for a while before souring on Amazon. Now I’m bought into the Kobo ecosystem, which is great in some ways but frustrating in others.

I’m curious if there’s a sort of DIY book cloud platform out there. I’ve come across a few, but they all seem to lack what, to me, is the killer feature of the Kobo/Kindle platforms:

Cross-syncing between mobile (iOS), ereader, and web reader

Most of the ones I’ve found can do this with some of those devices, but not all three.

I ask because I regularly hop between reading on different devices to the point that I avoid reading books that I can’t do this with (e.g. all my DRM free books, physical books, etc.). I’ve even re-bought books I already own in other formats just so I can have them inside the “sync loop” because it’s so much easier for me. I’d rather not have to do that though.

Are there any independent options out there that cover this use case? I primarily want to use it for DRM free books I got from bundles, as well as books that I de-DRMed from my Kindle. I would also happily buy a different ereader device that supports this (currently I use a Kobo Forma).

Meta note: wasn’t sure if this topic was better in ~books or ~tech — feel free to move it if needed!