It seems that the System Settings touchpad module (on KDE Neon 5.9) does not contain an option to turn the touchpad off (it does contain options to disable touchpad while typing etc.; there are also configurable keyboard shortcuts to disable / enable it - and these work well - but there seems to be no "global disable/enable" option anywhere...).

So... I want to disable the touchpad on startup (but without compromising touchpad hotkeys functionality, so that I can later turn it on). The question is - how?

I just tried to figure this out for my Thinkpad W520
in the options to disable/enable there's an option to disable while there's a mouse plugged in.
Under that is "ignored hardware" these things would normally disable trackpad, they're being ignored.
Remove one or all from that list and the trackpad will be disabled.