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 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

No for Whispersync. Kobo only offers synchronization for books purchased through them. On the other hand, KOReader does offer synchronization.