For anyone else interested in adding another 8 GB RAM to the existing, I can confirm that another RAM-module with CAS22, 1.2 volt works well to get dual channel 16 GB in total (2x8GB). The stock Hynix memory module is outdated and cannot be bought, but a standard memory like the "Crucial RAM CT8G4DFRA32A 8GB DDR4 3200MHz CL22" (available at Amazon) works with the pre-installed one. It all runs on dual channel 1600 MHz, so 3200 MHz in total. See below.
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