I'm happy with both my second hand HP Elitebook 840 G6 and the ThinkPad carbon X1 Gen 9 I have for work.
I think I like the HP one slightly better: function keys can be set to trigger either F1-F12 or to the actions drawn on them without the Fn key (and the Fn key swaps this). If set to the actions by default, F1-F12 are still automatically used when pressing a modifier key, and no action is on F2, which means I almost never need to use the Fn key for those and I can intuitively use alt+F4. That's not the case on the ThinkPad. It has a proper menu key (on the Thinkpad, they decided to replace it to screen capture, which is on FN+Right Caps on the HP). I like the metal feeling of the case and the feeling of the keyboard (but the ThinkPad is good on these areas too). Both have a touchscreen, and there are visible, diagonal lines on the Thinkpad's screen. Which is not very problematic, but better without. The HP has an Ethernet port, too. I think Linux works slightly better on the HP too: the sound automatically switches to the headphone when plugged, and switches back to the internal speakers when unplugged, on the same distro (openSUSE Tumbleweed). Though that might be some settings issue. S3 sleep works flawlessly on the HP. On the ThinkPad, it is not supported and indeed it does not work well. They decided to switch to whatever Windows decided to do with suspend, which does not really turns off components but put them in low power mode, which is a mess.
The ThinkPad is lighter, probably has a better sound from the internal speakers (though the HP's sound is correct too). The trackpoint on the ThinkPad is way more useful, you can scroll with it by holding the touchpad's upper middle button which is not there on the HP.
Both have a long battery life. I can recommend both.
I've not tried the KDE Slimbook 15, but it is a more expensive rebranded version of another model if I remember correctly.
No. Actually that was a main requirement for me. I hate noises. The computer is completely silent except when doing heavy computation or maybe long video calls in the browser (and then it's not that bad, though I would not notice that much because my headset blocks a lot of noises), and only if KDE's power setting is not set to power saving (though some things can feel a bit sluggish then). And the computer keeps cool.
The 840 G3 was similar in this respect (that's how I discovered the Elitebook, the lab had an agreement with HP at the time), except I had to turn off the secondary HDD I chose to take when tweaking the configuration with hdparm -Y.
Though the ZBook seems more focused on performance than the Elitebook, so maybe the Elitebook will not cut it if you expect similar performance.
The ThinkPad actually spins its fans more easily than the HP I think, but it is also more powerful. The HP's noise when the fan do spin is also less annoying than the ThinkPad, it's a soft blow.
I think I like the HP one slightly better: function keys can be set to trigger either F1-F12 or to the actions drawn on them without the Fn key (and the Fn key swaps this). If set to the actions by default, F1-F12 are still automatically used when pressing a modifier key, and no action is on F2, which means I almost never need to use the Fn key for those and I can intuitively use alt+F4. That's not the case on the ThinkPad. It has a proper menu key (on the Thinkpad, they decided to replace it to screen capture, which is on FN+Right Caps on the HP). I like the metal feeling of the case and the feeling of the keyboard (but the ThinkPad is good on these areas too). Both have a touchscreen, and there are visible, diagonal lines on the Thinkpad's screen. Which is not very problematic, but better without. The HP has an Ethernet port, too. I think Linux works slightly better on the HP too: the sound automatically switches to the headphone when plugged, and switches back to the internal speakers when unplugged, on the same distro (openSUSE Tumbleweed). Though that might be some settings issue. S3 sleep works flawlessly on the HP. On the ThinkPad, it is not supported and indeed it does not work well. They decided to switch to whatever Windows decided to do with suspend, which does not really turns off components but put them in low power mode, which is a mess.
The ThinkPad is lighter, probably has a better sound from the internal speakers (though the HP's sound is correct too). The trackpoint on the ThinkPad is way more useful, you can scroll with it by holding the touchpad's upper middle button which is not there on the HP.
Both have a long battery life. I can recommend both.
I've not tried the KDE Slimbook 15, but it is a more expensive rebranded version of another model if I remember correctly.