Custom instruments on macOS?

I'm currently using MuseScore 3.6.2.548020600 on macOS and would like to add a custom instrument.

I found the instructions for Windows at How to edit or create a new instrument which seems to document the instrument.xml file for MuseScore 1, but the format is slightly different now.

I located my instruments.xml file at /Applications/MuseScore 3.app/Contents/Resources/instruments/instruments.xml, backed it up and edited it with VS Code, and as a starting point inserted the following block of XML between the instrument definitions for octave-mandolin and shamisen:

<Instrument id="phin">
      <family>plucked-strings</family>
      <longName>Isan Phin</longName>
      <shortName>Phin</shortName>
      <trackName>Isan Phin</trackName>
      <description>Isan Phin (a 3 string mandolin from the Isan region of northeastern Thailand)</description>
      <musicXMLid>pluck.phin</musicXMLid>
      <StringData>
            <frets>15</frets>
            <string>57</string> <!-- A3 -->
            <string>64</string> <!-- E4 -->
            <string>69</string> <!-- A4 -->
      </StringData>
      <clef>G</clef>
      <barlineSpan>1</barlineSpan>
      <aPitchRange>57-86</aPitchRange>
      <pPitchRange>57-86</pPitchRange>
      <singleNoteDynamics>0</singleNoteDynamics>
      <Channel>
            <controller ctrl="32" value="8"/> <!--Ukulele-->
            <program value="24"/> <!--Acoustic Guitar (nylon)-->
      </Channel>
</Instrument>

Reopening MuseScore and going through New Score, General > Choose Instruments, I can't see "Isan Phin" under "Plucked Instruments" between "Octave Mandolin" and "Shamisen", and searching for "phin" matches nothing.

Have I got the instrument definition wrong, or is there a different process I should be following for macOS? I did kind of expect to be editing something in ~/Library/Application Support/MuseScore/MuseScore3 so that customisations don't get lost on version updates.