Too little coffee, but probably not calcified

Hello dear forum,

who has an idea on how to help my machine (DeLonghi ESAM 3500 S) that is noticeably struggling to make coffee?

It takes ages to brew and the coffee only comes out of the spout in drips. The crux is, when I descale the machine, I get one or two perfect cups, then it immediately decreases noticeably. I've been through this little game three or four times now....

Is it possible that I decalcify the machine just enough for one or two cups and then it's over again?

If it is a different problem, why do I achieve any effect at all by descaling?

I have descaled for a while with citric acid. Then I read that citric acid only dissolves lime but does not dissolve it and that it can therefore happen that crumbs clog the machine. That's why I now use a universal descaler based on amidosulfonic acid.

To descale I start the machine in test mode and pump the descaler through the machine. Always so half a cup, then 5-10 minutes break.

I hope someone has a tip for me, what I should look for ...