The reviewer that has trouble with Mac Mini said it has updated to Catalina and still have trouble with booting. So it wasn't Catalina that has compatible issue with his unit. If that's the case, you can easily google Catalina and samsung memory on Imac, you would find plenty of complaints. His probably have a bad memory, that's not unusual, just bad luck. Memory compatibility has little to do with OS and more to do with hardware.

I don't usually upgrade OS. i used it for DB, programming and mobile developing apps so OS compatibility on my apps is very crucial. if it's working, don't be a smartass and upgrade OS that doesn't benefit your workflow - that's my thinking. I also have a MBP and it's still running High Sierra. I don't plan to update my iMac unless having issue with applications for a year or two. I used mine for work along with photos and video editing.  128Gb is just right, i have seen my memory usage peaked 90+GB, 128GB is not overkilled for me. you can try 2 sticks of 32GB then upgrade when needed.