Hi Dan, I think you've gotten some good advice so far. Because your situation
sort of reminds me of where I was when I first started out, I had a look at this
thing (I'm assuming that it's the "Advanced 1500?") to see how it might compare
to my first (weird) workbench.

The bench's base does look a bit flimsy-looking for tougher hand work. In
particular, the dowel joints that hold the tops and bottoms of the trestles to
the legs are not going to cut the mustard. It really needs side stretchers (or
something of that nature) to keep it from racking front-to-back. That's an easy
add. The stretchers in the front and rear might actually be adequate (see below
for knockdown blurb). The legs themselves look just a tad skinny for planing;
they seem to be flexing at the end of Highland's assembly video. You could
laminate something to them to fix that.

They state that the top is 30mm (~1 3/8), beech, with aprons around the edges.
That thickness is actually quite similar to my first bench. What kind of weirds
me out is the way that the top rests on the legs. It seems that only the apron
rests on the trestle tops, with a sort of weird hollow between. With a benchtop
of that thickness, it seems that you'd only really be able to pound on stuff
around the apron near the legs.

I dunno. If I were in your shoes, and knowing what I know now, I would still
build my own. Perhaps relevantly, I made a curious experiment last year that is
a bit under four feet long with a 2.4" top:
https://galoototron.com/tag/auxiliary-workbench/
I put it opposite my main workbench "temporarily," and ended up using it a lot
more than I expected, especially after I made a "bench-on-bench" to go on top. I
was worried that the top wouldn't be thick enough, but that was unfounded.

This got me thinking that if I'd made a slightly longer version of something
like this for my first workbench, with a bench-on-bench, it might have been...
"better" somehow? Well, I dunno, it's not like I can go back and change it. And
I've had the same manner of space constraints as you until very recently.

So space constraints actually relate to knockdown benches, and here's my two
cents. A knockdown workbench can be just as sturdy as anything else, but you
cannot skimp on the size of your components or your joinery. Those stretchers
and legs need to be beefy, and tenoning the legs to the top is preferred even if
you don't glue them.

My main workbench is knockdown. But here's the thing--I've never actually
knocked it down for any of the three moves that it's undergone since I built it.
It took me a long time to guess why. I think it's because it's not very big (a
six-footer, and not terribly deep), so it's generally easier to carry and
manipulate through doorways and staircases than something that's more difficult
to flip upright and turn.