OP, your little boy sounds just like mine (2 years 2 months). My son had 3 single words at 18 months and has added 3 more within the last month -which was momentous for us, but obvs way behind the normal expectation for spoken language at his age. His receptive vocabulary, understanding (without visual clues), listening and attention on the other hand all seem pretty good, and everything else seems normal. I was a late talker myself, and went from a couple of words to full sentences pretty much overnight at around 2 and a half, so the people who tell you it may sort itself out in time are right - this can happen, and actually I think the odds are in favour of late talkers catching up spontaneously.
But the thing is, you can't tell which children are "just" late developers and which need intervention, so "wait and see" is playing the odds, which feels uncomfortable. Add to that the reality that SLT resources are stretched and waiting lists can be long - in this area it's 18 weeks from referral to assessment, and then at least another 18 weeks before any recommended therapy begins, so I would ask for a referral sooner rather than later as it may be some time after you ask for help before any help actually arrives.
It's worth asking questions, and finding out whether you can self refer. Here, the HV service guidance is to wait until 3 to refer if there are no issues other than expressive delay, but the SLT service's own guidance is to refer at 2 (go figure...) and when we called their helpline for advice they accepted a self referral straight away (currently on that long waiting list for an appointment). A hearing test seems a good idea too, and we're waiting for an appointment for one of those too, though to be honest will be astonished if it reveals a hearing problem as DS's understanding seems so good.
I agree that the stating-the-bleeding-obvious nature of the advice people often give is demoralising or annoying, according to mood at the time. I kind of wish we were those parents who have the TV on all day and don't engage with their toddler, because then there would be an easy fix.