Just after buying the iMac I got it to a local Apple Dealer and replaced the default 1TB HDD with a 256GB Samsung SSD + 2TB Seagate Combo. Now the Seagate died (after 6 years, I assume this is ok). So I replaced the Combo with a current Samsung 860 500GB SSD and a WD Blue 2TB HDD. No Issues so far!
Thanks a lot for the Guide!
Some note, I do not have the suction Caps so I was able to grab the Front Glass by applying my fingernails between Glas and Aluminium Body. That worked reasonably well too.