Not sure if this fully fits your needs but you might want to look at Wiki.js. You can self host it, has WYSIWYG editor available (as well as HTML and markdown). I'm not associated, just use it for some time.
Yeah, wiki.js is in the same space and seems to be pretty great. BookStack and wiki.js have taken quite different design & structure choices though so I usually advise trying out the demos of both to see what best fits.