Scored Monarchs are common at the moment, however Monarchs are also common at the moment.
Last 4 I have stripped for service all had scored shafts, 3 were within warranty and replaced.
I “think” yokes exasperate the issue by increasing side load. Metric shocks have increased bushing overlap.
Wear on one side of the shaft is common on Monarchs on Whytes.
Fox shocks seem better sealed, but would need to see some in long term use on the same bikes to say if it would resolve the issue.
Have you had a look for a new old stock float 216×63? I would be tempted to give a fox a shot if you are buying a new one anyway as the chances are a Monarch will do the same thing again in a similair timeframe.