I think you are in denial about the cars throttle shaft bushing being a factor here. Its a huge factor at all RPMs and vacuum loads. That much play in the shaft is not acceptable.

On a four barrel the front-drivers side bushings wear first. That's do to the extra load on the bushing from the linkage, throttle cable and return spring pushing / pulling in all directions. But on your set-up, I wonder if that particular bad carb was at the middle position at one time. That would explain the extra wear. Or maybe just the base plates was swapped around on a work bench, who knows.

I don't believe the shaft is plastic. But the end pieces or bushings might be black anodized soft metal. IDK Regardless, that one bad bushing must be replaced, better yet do both sides. The amount of gap shown in the video is equal to drilling a 1/4" hole in side of the carb. Its messing up your air / fuel ratio. You have "extra" air introduced below the throttle blades and above the blades, not good.

 

As mentioned before: I blocked the intake under the front carb with a sheet metal and it did no affect the jerking.
Only thing that changed was idle lowered some rpm. So air is coming in at the carb, blades and shaft.
It´s not good, no doubt, but it does no cause an issue jet. I guess it has to be worse to take effect.

I pulled the PCV out of the valve cover, that thing is sucking a lot of air, at least it sounds like doing so.
And when this line is blocked it has also no effect to my problem.

I will rework the throttle body in the winter time, but not now, this will take some time.

The front carb was always there, the throttle body and main housing are different from the center unit.

In a prior test I opened the secondary butterflies to get a different operating point of the primary.
There was a noticeable amount of air going in there, and this caused strong jerking when cruising slowly.

In my case, with the leaking shaft, the AFR is rich enough to compensate that.
As mentioned before it starts also jerking when I lean out the idle, maybe I can lean it more when
the front carb is rebuilt. But I did not try a lean idle with the sheet metal under the carb.

 

Update:
With the bigger gap of the plugs the engine ran better, but still not perfect.
I checked compression, all cylinders have similar and very good values.
Valve lift also ok on all valves.
After 100 miles the plugs look nearly like new ones. The leaner idle mixture
has a great effect on that. Never had such clean plugs before.

Last thing was timing chain slack, and that´s about 4 degrees at the crank.
Plus 2 degerees between cam and distributor which should be fine, because gears looking good.
Question would be how much this lets jump the timing, because the load
of the rotating parts pulls the slack out. But how steady?
I don´t how much the timing light tells in this situation, but the marking on the balancer stays solid at the same place.

Currently I´m not sure how to go on. I gapped the plugs a little more, will see if it helps.

The old distributor with the worn gear caused really hard jerking, that´s why I think slack in the valve train
could really be the cause of my problem. On the other side, the distributor itself is easy to turn, and likely to jump
around. I don´t think so about the cam at ~1700rpm.

I should measure the timing steadyness with an oscilloscope during driving, that would be the best thing to get an answer.
But when the jerk occures this will definately have effect on the timing and I could get missleaded. It´s not so easy...