Unfortunately there's a few issues on slim that you listed that doesn't makes it a good candidate, also unmaintained
Yes I was always thinking to change the greeter at least, just didn't found time for it yet
https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fnbh7d06nn5m01.png
This one looks amazing and perfect
BTW it is supposed that the login manager should support (in the future) wayland, since the world is moving to wayland (since years) so X11 will be probably deprecated in the future.
Entrance is probably not hard to move to wayland, but it has not an active developer / maintainer to work on that project, maybe @TheTechRobo would like to work on it, since he wanted to get involved in some development project
lxdm could another candidate if supports the multiple needs
note: if im not wrong, lightdm is not strictly required to be gtk, so the greater can be made in any type of interface 
if so, this can be the best option to pick, i just didn't had time yet to start investigating it
That looks pretty cool for a next retrowave build default wallpaper 
