In my opinion,
the throttle return spring pressure on a waterboxer engine is already
too high .
If the cable is all siticky in the forward white tube..
that may need cleaning and lubing ..
'all' vanagons need this in my experience. That tube is seldom cleaned
out, or the bell crank under the front floor lubed, or the cable where
it goes through the black outer part in back ..
'most' vanagons need that spiffed up , lubed and adjusted, in my experience.

your engine mounts would have to be really bad , or your throttle cable
really funky for engine movement to affect throttle.

how much side play is on your throttle shaft ?
Like with engine off, how far can you move the top of the shaft left and
right ?
I'll check a few of mine ..
should be basically no slop there.

I would just check everything ..
including compression and rocker arm screw adjustment , just in case
something is off there.

I see in your video that the shaft appears to displace sideways just a
tiny bit when you open the throttle.
If I get a chance, I'll check some of my vanagons' throttle shaft side
play.

Obviously, it started life out with no play or slop there , I think is
safe to say.

'surging' ..if it's really actually 'surging' I think of air , and air
flow not being fully right , and that fits throttle body.
If it's uneven in a subtle way I think Air Flow Meter.