I like the minimalistic presence it has (nothing extra unless you need it). I haven't really made a "huge web site" type of app with it but I did once tests with the templates (used PHP-view templates) and they worked. You install a template component (of your choice) via composer: https://www.slimframework.com/docs/features/templates.html and use it.
What I have done with it for real use is couple of REST APIs and for that purpose it was more than great (kinda off topic). Anyway you can easily install components for stuff like templates, caching, CSRF protection etc via composer when you need them.
What I would suggest is that OP goes through the "User Guide" provided in their page. It shouldn't take too long and if he feels it does not fulfill his prerequisites then dump the idea of using Slim.
What comes to boilerplate ready made "templates" for login / welcome page / lost password page there is plenty of examples in the internet and you probably have to fork them a bit anyway to make them work with framework/templating you chose to use.
I do not really know if Laravel/Zend or similar frameworks has ready made templates for that kinda stuff. But what I know they are not the lightest frameworks.
CodeIgniter is fine too.