how i set it up. im writing this for future self-reference. by virtue of presence of https://sarpi.penthux.net:

1. setup the sdcard with the rpi3 v15 sarpi .zip (.img didn't boot on me)

2. add a new root partition. i chose f2fs cause it's allegedly tuned for longevity

3. run setup. by default slackware forces you to install everything. don't fall for this trick, or do if you want to! meticulously chose the minimum required packages (this is helpful but doesn't list every package/dependency needed, https://www.linuxquestions.org/questions/blog/rkelsen-138375/minimal-slackware-installation-for-network-appliance-or-virtual-machine-38788/; my current slackpkg template is https://paste.rs/WdeYG, nothing seems broken as of now)

4. follow the guide carefully, https://sarpi.penthux.net/index.php?p=installer. there are minor changes here and there enough to end up with a broken install

done! probably not the best written. it's for me, should suffice