Well, I'm closing this chapter:
- Started in recovery mode, deleted the pending* file in C:\Windows\WinSxS
- run "DISM /Image:C:\ /Cleanup-Image /RevertPendingActions /ScratchDir:C:\Temp" but it failed.
Restarted, and SNMP service works, if I connect from another server I can set it up and it's now reporting, judging by the missing tabs in the service 'Server-RSAT-SNMP' is missing, and installation fails with error 3017 (restart missing, despite having restarted several times), also Server Manager shows "online; cannot get role and feature data" but at this point honestly I don't care anymore, we'll never instal another role in this server anyway and I'll never change SNMP settings, and if that need arises, I can do it remotely from another computer anyway.
In any case this seems like it'll prevent windows update, so I might open a new thread for that specific problem, the "install SNMP service" goal for this thread is fulfilled.
Thank you very much for your help and patience!