What is the failure mode of making an adapter to convert a 240 V outlet to 120 V
You can't just slap up a subpanel anytime you want to split a 240V plug.  However, with a NEMA 14, you can -- because it has all the right wires to feed a subpanel.
So, that's it.  You convert the 6/3 cable run into a &quot;Feeder&quot; to the subpanel.
As feeder, 6/3 cable is rated for 55A. They don't make breakers that size, so the feed breaker (in your main panel) gets bumped to 60A.
Eaton CH and Square D QO have perfectly compact 8-space panels, and that's the way to go if you are cramped for space.  Otherwise if you match your main breaker's type, you can reuse that 50A breaker.
You don't need a main breaker in this panel, unless the building is detached from the house (and then you do).  If so, either choose a panel with a main breaker, or those 8-space panels can have a main breaker added via backfeeding.
Install a 50A breaker to feed that EVSE socket.  Then install as many 15A or 20A breakers as you please to supply receptacle loads.  You can safely use two 120V circuits at the same time while charging. Realistically due to the 80% rule, your EVSE will actually pull 39A and your random power tools will pull 12A per leg.  So you're at 51A which is not a challenge for the 6/3 or the 60A feed breaker.
You can't overdo this.  If you're really going to town and running the saw, dust collector, compressor for blowing sawdust away, shop vac for your apprentice to pick up sawdust, etc. -- then the EV is certainly not in the garage at this time!