The editor problem
M4A holds AAC audio, and AAC is patent-encumbered in ways that make it awkward for open-source software to ship. That is why Audacity, in a default install, cannot import an M4A or an MP4, and why people keep needing this conversion for recordings their own phone produced. MP3 is old enough that everything reads it.
Trim first
Recordings of this kind usually have dead space at both ends — the walk to the table, the goodbyes after the meeting. Cutting that out before converting makes the file smaller and the conversion faster, because only what you kept gets encoded.