What's the self hosted ecosystem like for these? Let's say I have my own ebooks and audiobooks. Is there whisper sync like features?
I just set up Bookbridge (https://github.com/cporcellijr/bookbridge) to do this. It's so new that I can't even endorse it as I just don't know if it works well yet, but this is at least one solution.
No for Whispersync. Kobo only offers synchronization for books purchased through them. On the other hand, KOReader does offer synchronization.
I have no idea what Whispersync is but Kobo works very well with Calibre or Grimmory. You can change the store URL to your own Calibre(-web) [1] or Grimmory.
Some of the Kobo also support audio books. All of them have microUSB or USB-C to sync, some got Wi-Fi or Bluetooth as well. Mine even has a mic for some reason.
There's a plethora of hacks for devices like the Kobo (reMarkable is another one). For example, see [4]
[1] https://calibre-ebook.com
[2] https://grimmory.org
[3] https://koreader.rocks
[4] https://github.com/RiccardoSilva42/Kobo-Hacks-Comp