One important thing to know first is that MIDI, unlike tracker modules, doesn't really have a fixed concept for changing the volume of a single note. MIDI volume (CC 07) messages affect all notes playing on the same MIDI channel, meaning that you are effectively limited to a 16 note polyphony (one per MIDI channel) when using MIDI volume for changing note volume. The only type of MIDI message that would allow addressing individual notes on a channel - polyphonic aftertouch - is not well-supported by both hardware and software synthesizers and is typically not even used to modify volume (but on some synths it can certainly be configured that way).
With that out of the way, the volume command handling option you found is a bit of a hack and indeed only affects set volume commands (vxx), not volume slides. PC notes aren't going to help you either because you want to talk to a MIDI synth (PC notes only work with plugin parameters), but you can set up some MIDI macros to do the job. I have attached an example with two variants:
On channel 1, MIDI CC 07 is sent, taking the parameter value of command Zxx or \xx. With \xx you can do smooth slides, but not the same way as c0x (you only specify the target of the slide, not the speed and direction). On channel 2, actual volume slides are used and the MIDI macro instead uses the letter "u" to evaluate the CC event's parameter based on the current tracker channel volume. As a consequence this approach occupies both effect cells if you want to update the volume. Not optimal but it's pretty flexible (as it will also work with instrument envelopes). (Side note: The slide on channel 2 starts at volume 1 because a zero-volume note- on event is equivalent to a note-off event in MIDI).
I know it's not optimal, but hopefully there will be a more intuitive way in the future that doesn't eat a whole effect column.
am I right to presume these are provided by Windows/laptop soundcard
Indeed, GM.DLS (and by extension OpenMPT's default MIDI library) is shippsed with Windows/DirectX. Soundcards haven't really included any ROM samples in the last 20+ years.
There's plenty of better-sounding soundfonts that you can load into OpenMPT instead.
