If using Calibre, I suggest convert your books into Kindle-native *KFX format. Also, filling out some metadata in Calibre even enables sync with Good Reads for sideloaded books
I wish there was a simpler and more native way to do this. Because I found a way to get a notification on my watch if I forgot my phone, but not the other way around, and now I'm trying to figure that out.
Calibre can convert to *.kfx native Kindle format