Neither Ubuntu 16.04 nor a Xeon processor are required.
"At the moment i'm writing all witnesses are using the version v0.19.2"
This is sort of true (in reported version numbers) but currently* witnesses should be building/running from the stable branch right now, because there has been a denial of service fix that is available on that branch and not the v0.19.2 release.
*This is subject to change, for potential future readers
You briefly mention running screen but there's no explicit mention of keeping steemd running after logging out of the terminal.
I see you are running the v0.19.2 release. what do you think is the best solution to solve the DOS problem?