I am getting close to reinstalling Debian ( shades of Windows) !
Debian sid is unstable by name mostly because of package churn and large
transitions like this. Unfortunately, it's not intended for inexeperienced
users - you are expected to be able to resolve most problems yourself - "if
it breaks, you get to keep both pieces".
Lots of folk go to use sid because they want to be very up to date when
the boringness of Debian stable with security support and defined upgrade
paths is more appropriate for them.
If something is going to update 80 packages at once, try manually using
apt or apt-get to install five at a time - take it more steadily and take
notes of any package breakage, maybe.
Look in the Debian bugs database / the Debian package tracker to see
what bugs have been recorded against lightdm recently.