OK, then...that did not go any better at all.
Failed to boot into 4.20. It is simply stuck in black screen and blinking cursor. Like before. So the update of Grub did not improve anything. I think it has something to do with modules not installed to support LTS (or something, no expert). I think the change has to be made before any drivers are installed (like gfx and such). I did it on my MacbookPro and at that time it worked fine.
So I booted back (after reset) into kernel 5.x and after an excessively long boot time 5+ minutes I got to use the desktop. During this time both udisks2.service (whatever that does) and dropbox used the previously mentioned drive (with /home). During this time the system was unusable since it basically took all power to do this. When checking with "systemd-analyze blame" the udisks2.service took 4 min 5.882s to run. After that fsck ran up 17.795s on my drive. The rest was like previously mentioned. So it all points back to the bloody drive again. So yeah, I'm reading the message on the wall. Time to recover while possible. Nothing is screaming "I'm broken" but still all clues point to this drive what ever I do. So it is best to face fact. Something is wrong. So I stick to the plan. Copy data, rip out old drives and replace them and then re-install Solus 4 and lock down LTS kernel. Then bring all back up (with improvements naturally. One can not help learing stuff during a year). Thanks for all help. I will do a final update post when it's done (it will take a while) to follow up on the situation (if it helped and so on). I will also put in a new 4 slot 2,5" dock with hot swapable slots so I can try out other distros while still using Solus as my main. I'm VERY curious on trying out MX-linux on my main machine (i have it on the sidekick and it works like a charm). MX is my backup distro right now. Anyway...onwards and upwards! :-D
