Via any route, boot ends on white screen

Macbook pro 15", late 2011. Running latest High Sierra. Upgraded to 16 GB RAM.

All kinds of boots ends on a white screen before finishing.

I've tried "everything":

• resetting SMC, NVRAM and PRAM
• safe mode
• single user mode (with different commands)
• verbose mode
• recovery partition
• internet recovery
• replacing the SSHD with my previous HDD (with high sierra installed)
• booting from USB with full high sierra installer
• booting from external HDD with full high sierra installer
• draining the battery completly

The Mac typically boots, Apple logo appears and progress bar starts to fill, but before it is full: stuck on white screen.

Option-start let's me select boot disk and network, then as previous paragraph. Internet recovery downloads its stuff, then white screen.

Verbose and single user mode seems to load fine, no display issues. But on exit or reboot, it eventually ends in a white screen.

I did access the SSHD from another mac via target disk mode. Ran disk utility first aid on the SSHD, and my mac then booted once, but froze. Highly unstable performance.

Before all this, I had updated MacOS with security update january 2019. After a few days, the mac froze, somehow Chrome was involved, but that could be random. Display would flare with horisontal lines, screen image being displaced halfway and coming in on the other side. Theese screen issues does not appear during all the different boots.

Some years ago, the motherboard was replaced in an Apple program addressing an issue with theese Macs.

What to do?