What are some synonyms to 'tenure' when the post is informal? Can I say 'during his deployment' or 'during his assignment'? Lexico doesn't support this view, the words don't have a sense "a period of time". What can I use instead then? I don't like 'during his service' because it doesn't fit the context: the man in question is more of a supervisor than a servant.
As the investigation found, no later than in June 2014, a GRU officer was deployed to Luhansk where he served as a military advisor. During his [deployment], he coordinated military activities of pro-Russian insurgents and mercenaries of the so-called Wagner Group.