Yes, constant removal and insertion on 14-50 receptacles is not great (especially on low end ones like Leviton units which are extremely common).
Inserting and removing the adapter on the UMC is likely bad too, I doubt that custom plug is rated for many cycles.
Would you rather put the wear and tear (if you are going to insert/remove all the time) on the $10-40 14-50 receptacle, or on the several hundred dollar UMC?
I personally am a massive Wall Connector fan, but that is just me. A second UMC would also work fine it sounds like. A Corded Mobile Connector could also work.
I would not put my own pigtail on a Wall Connector as you miss out on the thermal sensing abilities of the Tesla factory options. Receptacles fail a lot and so that thermal sensing ability is important.