Thanks @Xrott - that explains it

Thank you - repeated the exercise with your comments in mind, and can confirm this is how Inkscape behaves.

There seems to be no "Text units" in Inkscape 1.2 / Preferences / Tools / Text, and it seems to be always "px" now?

Well, it turns out, this works ONLY for nodes that are expanded in the XML editor; if you have thousands of nodes within a parent node which is collapsed in the XML editor, it will not be found, neither by id nor by label. What's even funnier, there is no "expand all nodes" button, and you cannot even use the keyboard to expand the nodes - you HAVE to click the little triangle, one by one, for each of the thousands of nodes! How convenient! :)

Many thanks @KuoHilas - great answer; now I recall I've seen this approach elsewhere years ago - and had promptly forgotten about it; good to have it in writing now!

Thanks @BillyKerr - was not looking for chamfered corners, just an easier way to edit diagram arrows (which I manually edit anyways)

Excellent suggestion, "how come I didn't think of this" :) - many thanks, much appreciated!