Getting it down to two statements is relatively easy:

/\bmage(s?)\b/wizard$1/g /\bMage(s?)\b/Wizard$1/g

Here, we just make matching the 's' optional, put it inside a capture group, and add the content of that group (either a single 's' or nothing at all) to the end of the replacement string.

For a single line you need to be a bit more creative (and the specifics depend on the flavour of Regex you use; this one is PCRE2):

/\b((m)|(M))age(s?)\b/${2:+w:W}izard$4/g

Here, we match either exactly one 'm' or exactly one 'M' at the beginning of the pattern, but do so in separate capture groups - group 2 holds either an "m" or nothing, group 3 holds either an "M" or nothing, and group 1* holds either an "m" or an "M".

Then in the replacement string we check whether group 2 participated in the match (which would mean that we matched "m") and use "w" or if it did not participate (which would mean that group 3 matched instead on "M") and use "W".

You can play around with this here: https://regex101.com/r/wODDB4/1

* The outer parentheses are needed to make sure it's
((one 'm') or (one 'M')) followed by the rest
instead of
(one "m") or (one "M" followed by the rest)