Looks a lot like the other video. I still can't see the lag/jump on the brew boiler gauge. All I see is a normal ramp to max pressure.There might be a little movement after that, but the camera is too far away to see it. Aim the camera at the brew boiler gauge on the front panel and keep it there. Get in closer if you can.

I wouldn't rule out that more torque will completely close the fitting, but better if you leave that to the tech. After it's fixed, make sure you get an explanation of what was wrong and which parts were replaced, if needed. Before the tech leaves, run the machine from full off to full warm-up and through some free flow and brew cycles to make sure the leak is gone.

Since you have a tech coming, it would be a good idea to make sure there are no other leaks. Remove the both side panels, the cup tray and the top panel and carefully inspect all areas of the machine for leaks. Leaks are uncommon, but new machines are probably more vulnerable due to possible loosening of fittings during shipping. The consequences of a leak can be quite severe because the logic board isn't well protected and it's very expensive to replace.

Since you slowed or eliminated the leak, I think you can run the machine until the tech gets there, but do it with the covers off so you can keep an eye out for any water accumulation under the fitting or anywhere else.

On bleeding, my GS/3 is plumbed in, so I don't run the pump to refill it. The 3-4 BAR line pressure takes care of that, albeit quite a bit slower than if I were to run the pump at 9 BAR. I use line pressure because the water doesn't spray out quite so violently when the group is full, like it does with the pump. I don't drain/refill very often, so the long fill time isn't a problem.

FWIW, the reservoir works well, but if you can plumb-in the water supply it'll make a world of difference to your enjoyment of the machine. Even better if you can plumb in the drain.

Interesting. My lag/jump was never that small, but the way you describe it is exactly the same. It only happened during free flow, never during a shot or while backflushing, and the sound of the water always changed after the jump. More of a rushing or whistling sound after the jump. Ever since I installed the new TL-30 tube that moved the gicleur location, the lag/jump is gone and I hear the post-jump sound right from the get-go.

All of the fittings in the GS/3 are British Standard, which don't require thread sealer or tape. They seal with a copper washer, a fiber washer, a flare fitting or, as in the case of your leaking fitting, a nipple that seals against a concave opening. In contrast, non-flare and non-compression NPT fittings typically used in the USA do require some sort of thread sealer.

There's no harm in using Teflon tape to temporarily fix a BS fitting that's leaking. I've done it myself while waiting for the proper washer to arrive, and I know of at least one professional espresso machine tech who uses it on fittings installed during pre-sales setup. However, best practice is to determine why the fitting is leaking and correct the problem. In your case, I think it's got to be some debris keeping the nipple from fully seating against the concave depression in the tee, or it's a defect in the surface of the nipple or concave depression, or it's a bad solder job on the nipple.