I already received help via Discord for part one of the issue:
Problem 1 (solved via discord) adding 19 mods to the steam workshop page after server installation - no mod was downloaded. Current version: v188.8.131.52, built 14/05/2019 14:19 limits the number of mods to 9. After that, the update string gets too long and throws an error. Adding 9 at a time successfully downloads them all.
Problem 2 (open)
SteamCMD downloads the mods to: /home/AMP/.ampdata/instances/
There they remain and the server does not pick them up during/after update or startup in order to copy them to: /home/AMP/.ampdata/instances/
Help is appreciated (will post an update here once I got it running).