Excellent guide! It took me about 1.5 hours to complete the entire RAM+SSD installation. The iMac has massively improved and feels like a new computer: way way faster than before, with a 1TB Samsung SSD, 32GB of RAM and a brand new installation of macOS Catalina. Be very careful with the I/O ports, especially with the USB-C/Thunderbolt ones: the alignment is critical, as well as the correct position of the logic board in relation to the case. Even a slight misalignment can impede the USB-C plug from fully entering the port, making it hard or impossible to establish a connection.

The bottom left screw was covered by a protective plastic adhesive circle that had to be removed to access the head of the screw.