Is it 'medium-to-long term' or 'medium to long-term'?

Many writers would use a suspended hyphen.

Medium- to long-term.

You're allowed some discretion on this matter, as reputable writers are not entirely homogenous in their hyphen usage. Between the two that you suggested, the second seems to be more defensible. Writing "medium-to-long term" is probably not achieving what you're trying to achieve with the hyphenation.

The challenge you're really facing is combining the phrases

Medium-term

and

Long-term

The logic behind the suspended hyphen is that it preserves the prefixed nature of both words (medium and long).

However, searching reputable articles on Google reveals that plenty of writers opt to omit a suspended hyphen and simply write "medium to long-term." For example, in the title of this scholarly abstract:

Medium to long-term efficacy and safety of oral tacrolimus in moderate to severe steroid refractory ulcerative colitis.

In your particular case, for a resume, I would recommend using the suspended hyphen, if only because it shows an attention to detail and an intentional writing style, but I don't want to delve into opinion too much.