something like FlexBar
 

I would like to have some AddOn similar to FlexBar where I could position each button and also label it.

Not currently possible. May be in the future, we don't know yet. Sorry.

I just would like to manage the attack skills of my character with the Num-Nock keys. And I would like to see on the buttons which key is for what skill.

The buttons that are mapped to hotkeys (with the key mapping menu in options) will display the keymap on the button once you place an action on it.

By default, bars 1, 2, 3, and 4 all are mapped... to no modifier (ie. 1 thru 0 and the - and = key), and with modifier (alt+, ctrl+, and shft+). On the hotbars they display as either just 1 thru = or C+1 thru C+=, S+1, A+1, etc.

So, by default, the hotbars do show the shortcut keymap as long as there is an action bound to that slot.

Here's an example shot from "Anduril Reforged." It shows the unmodified hotbar, shift and control, as well as a couple slots from an unmapped bar and some modified keymaps (such as the S+E, Q, and E on bar 1).

If you want to use the 9-key (numpad) for your skills, try the key mapping settings in options. Set the hotbar slots you want to the numpad keys you want to control them. I'm not sure if the numpad is recognized differently than the normal keys, as I've never tried, but give it a whirl.

Ohh but do I agree with the hopes that one day this will be possible. Just imagining the fun of those reaction skills popping up in specifically placed spots of my choosing for my own playstyle comfort.

/drool

Yes indeed.

Bars
 

What I really really really want is to be able to make at least ONE bar of my choosing swap to another quickslot bar when I stealth. Maybe someday...

How do I make the C+1, S+1, etc. Bars to be always visible?
They always vanish when I close my baggage.

use the settings in "quickslot" in the options menu

This isn't really a customizable UI, it's a skinnable UI. That means you can change the decorative graphics, but that's about it.