Hiya Martin, for using (and updating) slackbuilds I'd send you over to this site, which is based on -Current. Works very well and is updated separately; I usually do it once a week: https://github.com/Ponce/slackbuilds
Slackpkg+ packages are updated in the regular update proces, so that should be fine. Also multilib. Check your /etc/slackpkg/slackpkgplus.conf to see if they are all included. For me using grub doesn't make a difference, with a new kernel grub gets updated too.