Thank you Pressino for your input!

I have read at quite some places that "donuting" is actually a bad thing which shows that the edges were either not properly tamped or there is less coffee there due to suboptimal distribution. I understand that center start is also suboptimal, and the ideal situation is when extraction starts throughout the whole surface at the same time (which I was not able to produce yet, maybe only by chance).

Here is a video of a shot ground one click finer. As this is 18g in 36g out in 47 seconds, I think this is already very slow. Should I try it with finer grind? I think it will just choke the machine.

I am using the K-plus.
With the built-in grinder consistency is impossible (too much variation in the dose)

I think I might have figured out why my shot starts in the center: I am using a dosing funnel which sits inside the portafilter, so when I do my main WDT a ring at the edge stays with less ground coffee. After I remove the funnel, I carefully even out the surface with the WDT tool, filling the edges, but that coffee comes from the center, probably leaving less coffee there.

Next time I will try to do the whole process without the funnel, and see what happens.

dusangield wrote: Here is a video of a shot ground one click finer. As this is 18g in 36g out in 47 seconds, I think this is already very slow.

How did this shot taste?

it was more on the sour side rather than bitter.

However recently I think I am a bit overloaded with espresso, so nothing feels good really.

With "free hand tamping" I was not able to get consistent results, so after some experiments I seem to settle now with a no funnel, WDT, no OCD, no tapping then palm tamper set to fix depth which makes the tamper sit on the edge of the basket, providing level and consistency (so far). With this depth I need to apply considerable force on the tamper, so maybe this is not very far from an ok tamping force.

Looking at the raw numbers, it was a 18.1g in, 35.2g out in 27 seconds shot.
Tastewise it's quite sour without too much other complex tastes. And just small hint of bitterness.

What do you think about the visual appearance from diagnostical perspective?

According to link the large bubbles are caused by drastically cold machine or brew time under 20 sec.
In this case the brew time was definitely over 20 seconds. I don't have 100% confidence about brew temp, but the machine has 2 built in thermometers(boiler and grouphead) and both of them shows 92-93 C values. I have also measured the flowing waters temperature which went up to 93C as well after 6 seconds(this is a slow and cheap thermometer, but together with the 2 built in thermometers, I am fairly confident that the temperature is not drastically low.

Is there anything else which might cause such bubbles?

(Since then I have raised the temp to 96C, brew time went up to 34 sec with same puck prep and output weight, the taste was still dominated by acids, without signs of sweetness or bitterness.

Then I have raised time to extremes (40 sec) output was 56g, the taste was both distinctly acid and bitter without sweetness nor nice coffee taste.)

Any suggestion how may I progress?

The shot looks like its running fine. Don't be afraid to have shot times over 30 seconds including up to 40 seconds if the shots taste right. Based on what you've said about trying longer times and ratios I would be wondering if you're falling into the common trap of confusing sour and bitter. It's often a problem for people new to tasting espresso. Espresso always extracts flavours in the same order. Acidic, sour, sugars, bitters. An uneven extraction will give you a combination of this.

In terms of temperature here is a way to see if your group head water temperature is accurate.

Profitec Pro 600 Owner Experience

The flash boiling test is brilliant, hats down to people coming up with the idea!

Unfortunately the highest temp which can be set on my Oracle Touch is 96C, so I think I cannot perform that test. I am afraid I will need to bring this machine to a service center to have it's temp and pressure checked.

It is entirely possible that I confuse sour and bitter of course.

For me sour is the taste of lemon juice. With the current blend I am using (medium roasted Brasil) no matter how I manipulate brew parameters, I feel the lemon juice taste as soon as the coffee is in my mouth.
Bitterness is not always there, tends to come with longer times, higher temps, finer grinds. I start to feel it at the back of my mouth when I swallow the drink.
In my understanding it is expected to have the bitterness come with longer/hotter shots. What troubles me is that these sour and bitter tastes never complement each other creating a nice balanced drink with "nice coffee taste", but the result is either a sour drink or a sour and bitter drink.

I mean probably sour and bitter will never compensate each other, but they would need sweetness to be compensated, and that's what is missing. Probably adding sugar would help a lot.

And many times I can smell instense sweet smell of the puck after brewing. Not always though and I was not yet able to figure out how to reproduce this intentionally. But that sweetness never gets into the cup, or at least I can never feel it.