iMac nearly died - Power-On not possible but LED1 is on

About 2 month ago i replaced the SATA-harddrive of my imac 21,5” 2017 with a SATA-SSD. It worked nearly fine in this 2 month, only crashed 2 times (before my Harddrivereplacement it hung sometimes). But then about 2 weeks ago it dies in the middle of work, and doesn’t come up again, since then.

I opened the Imac to see, if i can see any problem, but i can’t. When i plugin the powercord the LED1 starts lighting, and that’s it. When i press the powerbutton, nothing happens, no fan, no further LED tried to do anything, not a small sign.

One thing that i noticed is, that when i first push the powerbutton and keep holding it and then plugin the powercord, the LED1 first lights up when i release the button.

I checked some cables inside, I tried to leave the powercord unplugged for a day or pressed the powerbutton for a long time, nothing helped.

So what do you think is broken: Mainboard or powersupply or do you have any further tips? Maybe the RAM? Is it worth to unscrew it that far? Has anybody similar Experiences?

Greetz Benni