Stable/unchanging (zero maintenance for several years, no new bugs appearing out of nowhere), as minimal or bloated as you want it to be (made my own custom live/installer ISO which means I can start fresh with my environment already set up in a matter of minutes), huge number of pre-packaged software, good documentation, identical base distribution across my servers and desktops, mostly sane defaults, community-backed (no or few corporate interests driving the project to shit... look what it did to Ubuntu/CentOS...).

Other Debian-based distributions don't bring anything of value to the table for me, I'd rather use OG Debian.

I use Debian as a desktop both for work and personal use because I am a boring person that doesn't need any surprises. I have been using it since woody after moving from Mandrake Linux

I used Debian on my desktop and laptop for many years, eventually I felt like I was spending too much time thinkering. I switched to Ubuntu 20.04 LTS and everything works great. But the snap crap is getting in to my nerves so I'll probably switch back to Debian next time.

These (Mint, Zorin, etc) and Debian are basically the same. You can use either of them and have mostly the same experience once you finish setting up your system. Exception here goes for Ubuntu that forces Snap. I personally go with a minimal installation of Debian and add what I need along the way. If you don't want to set things up thoroughly, you should use a distro you think has the most functionality out of the box.

What makes Debian unfit for Desktop use? Nothing. Debian is as good a Desktop OS as Mint, Ubuntu, Zorin and any other distro out there. It isn't just for tinkerers. Perhaps it doesn't have the amount of new user friendly features but I would rather use an OS that pushes me to learn than pushes me to be complacent and also pushes me to seek the 1-click options rather than spending 5 minutes to set something properly. Debian might not be everyone's cup of tea... but for those who it is their cup of tea Debian will never disappoint.

Stability

And neither of this distros are good with stability tbh

I typically install my own window manager, so Debian tends to to be perfectly fine for a base install for me.

I used Debian years ago, but switched to Ubuntu. After using that for many years, I got frustrated with Canonical forcing snaps and switched back to Debian a few months ago.

Because I'm a junior sysadmin. I need to learn all the stuff as fast as I can.

Imo, why ask the question with the assumption "is based on a server-oriented distro" when "dektop distros" such as Ubuntu, Linux mint Zorin are then using debian? It is a bit conflicting isn't it? it all boils down to personal taste, if you like ubuntu, use ubuntu, if you like linux mint, go use that. If you want to use debian or arch or fedora, you know...

For me it is easy enough, stable enough, bleeding edge enough (testing/sid) to tinker around without invading my machine with stuff I don't necessary want

Well,

For me its simple: i use Debian for servers, so who not for Desktops either. And personally i dont consider Debian a "server oriented distro" perse. for me its basicly a kernel with userspace software. You can make it a server if you leave it after a minimal install, or you can make it a desktop if you install a desktop environment.

I use it for both. It's rock solid and once setup and customised to my liking it's low maintenance. Basically I wanted a stable Ubuntu. I wouldn't say it's a server orientated distro, it's good at both.