Probably need to add "nomodeset" to the startup
Hold down the shift key when booting starts.
You will then get a grub console mode menu. The top option should be selected. Press 'e' key to edit that line.
Scroll down to the "Linux /boot/vmlinuz- your kernel-root=UUID.." line. That is the line...
Generally tap the F12 key at startup then you can select the USB drive to boot from - some makers like HP use the F9 key - the BIOS also has boot-up settings
My Expirion Linux has no systemd but uses SysVInit since it is Devuan based - personally I think SysVInit is a little faster then systemd since it uses less RAM - you can argue which is better all day long - but I think in the end it just a personal preference thing - whatever works for you
The only other thing my limited knowledge of Slackware is to reinstall KDE with the --reinstall option
Package Management for Slackware PDF
https://docs.slackware.com/howtos:slackware_admin:how_to_use_slackware_installing_software?do=export_pdf
Hope this helps
My guess is when you deleted the partition and therefore deleted the boot files along with it - I would suggest reinstalling manjaro to get the boot files back where they belong
See this guide on how to remove linux from dual boot- https://www.xda-developers.com/how-remove-linux-dual-boot/