I've been trying to endure debian simply because it installs on my mac however it's quite bad and I would like to copy over Slackware onto that drive, which boots perfectly fine i.e. the EFI setup is perfect. Is it possible to replace debian in this way?

My only experience with this is a roommate owned a Mac Book Pro which had some annoying problems that escalated to the point where OSX became slow and troublesome and asked my help oddly enough since I know little about Macs beyond history. I was surprised and please to see that Slackware Live booted and ran very nicely, even snappy, on the Mac. Since the bootloaders available on Slackware Live are identical to the Install Disk, I can imagine no obstacle past that point if what you mean is a new, clean installation. I do imagine it would be wise to first prepare the disk partitions at the very least as a failsafe since I can't recall if both Syslinux and EFI boot worked. I'm only certain that EFI did since the little "EFI" labeled icon came up and I remember that.

I do not believe that there's another way to migrate from Debian to Slackware other than a full backup of your data then to format your system partition and do a clean Slackware install.

Debian and Slackware are really different Linux distros and dumping the second one files over is a perfect solution for a full mess ahead.

I agree. Regardless of how you migrate from Debian to Slackware I would back-up all of the data that you can't afford to lose before you start the migration process.
I also think that a clean install of Slackware would be the way to go.

If by replace you mean substitute, then as long as Slackware runs on the Mac, yes. Grab a copy of the Live ISO to test against you system.

If by replace you mean convert from Debian, then no.

Either way, the Debian partition will need to be wiped before installing Slackware. Before wiping perhaps save the installed package list to help install the Slackware equivalents. For example, dpkg -l | grep ^ii | awk '{print $2,$3}' > some_text_file.txt.

Not asking for distro bashing, but I'm curious what is being bad for your user experience.

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Well... I neither endorse nor recommend this, but I have done it in the past. If you have some space left over at the end of the disk (or can create some), install a minimal Slackware environment onto that partition with this:
Configure Debian's bootloader to boot that minimal installation and ensure it all works.
Once that's verified, make sure you have at least the L and N package sets (or everything, if there's space) on a flash drive or something that you can access from that minimal installation. Boot that minimal Slackware, repartition the entire disk (except for the small partition from which you'll be running), install the full Slackware to the disk, install Slackware's bootloader over Debian's, and go from there. You can use the small partition as swap or something after that.

EDIT: you could technically skip the repartitioning -- just "rm -rf" the stuff on the Debian partition(s) and then install to them normally.

The backstory is basically this: this is how I "upgrade" remote servers on the rare occasion that an upgrade has to occur AND it has to be a clean installation AND I cannot get timely access to the physical premises. It's only happened once (and I hope it never happens again), but sometimes we do what has to be done :-)

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Trim partition, install Slackware along and then decide. Maybe your Debian dislike is temporarily only - and maybe Slackware would be as much disappointment as Debian.