When the espresso begins to pour it takes about 3-5 secound to reach the right presure, what can I do? Hello I have some problems when trying to get a perfect shot of espresso

The problem:
So when the espresso begins to pour, the pressure is not high enought, and the color is dark. It takes about 3-5 seconds to reach the right presure and the right color. 

So what can I do to get the right pressure when the espresso begins to pour out? 

The equitment I'm using is:

Machine: Sage Barista express

Grinder: Mahlkönig Vario grinder (Right now the grind size i'm using is 3.d)

The beans roasting date: 19/06.

I'm using about 16g of grind coffee.

I'm Tamping at about 15kg

The extraction time is about 22.4 secound from I press the 'double shot' button.

What I have tried:

If I try to use a finer grind the pressure will just get way to high, but still use about 3-5 secound to reach espresso range.

If I try to use a coarser grind the pressure will never reach the espresso range.
This is completely normal, this is the pre-infusion phase.

Pre-infusion is used to wet the grounds before starting ideal espresso extraction. Wetting with hot water helps the grounds to release initial CO2, therefore enables more and even extraction surface. It somewhat resembles blooming in pour-over manual filtering.

As a result, you should feel lucky that you have this feature on your machine. By the way, I take a look and found this explanation on Sage's website:


  Pre Infusion Function:
  Gradually increases water pressure to gently expand grinds for an even extraction. 


And a final note to enhance your cup of espresso...

As I can  see, you end up in 22.4 - 5 = 17.4 seconds, which is pretty fast. The common practice is 1 ml in 1 second; therefore you should fill a 30 ml espresso in 30 seconds (with double spouts a doppio will take the same amount of time 60 ml in 30 seconds.) I recommend you try to fix timing first to see if you can get better results.