Well spotted, thanks! I've merged all but the encryption tags. "disk encryption" definitely is an encryption – but encryption is not necessarily disk encryption. There's also FBE (file based encryption), text encryption (e.g. using GPG), encryption in messengers (e.g. OMEMO in XMPP), etc.pp., so IMHO we should better keep the two separate.
I went ahead and renamed stock-ui to vanilla-ui. I've also updated the tag wiki and the handful of questions that were using the old tag to use the correct terminology. There did not appear to be any questions that were using stock-ui to actually refer to a "stock" (manufacturer default) interface, so the tag is unused at this point.
recording should no longer be used; I've retagged the handful of questions that were using it with more specific variants to disambiguate it. It is not a synonym of anything at the moment since the more specific tags will appear in the tag autocomplete anyway.
We already had external-display so I merged monitor into it and created an external-monitor synonym.
I think monitoring needs some work though, it's too vague — there are lots of unrelated things that you can monitor. If everything is referring to process monitoring, perhaps we should rename the tag to process-monitor?