The biggest problem with battery life on a Galaxy Watch may not be a full battery. GWs use inductive charging (wireless charging). As such the battery only "sees" the amount of power it needs to fill the battery, not overfill it. When the battery is full and still on the charger it gets a trickle charge, inductively.
What does kill a battery is waste heat from inductive charging. Inductive charging provides energy from the coils in the charging puck to the coils in the watch to charge the watch's battery. Things like an undersized charger plug may limit the available current to the charging puck causing the watch to draw more power than the puck and the plug can provide causing the puck and the watch to heat up. Improperly seated pucks can also cause inductive heating. When the puck and the watch heats up the battery heats up. If it gets really hot it can dry out the electrolyte in the battery diminishing its life and charging capacity.
There are good tips on this page to help prevent heat related damage to your Galaxy Watch battery. I would encourage you to check it out.
BTW, whenever I charge my GW4 I wipe off the back of the watch to remove any dried perspiration or not so dried perspiration to avoid increasing the air-gap between the puck and the watch. An ever so slight gap like that can cause increased inductive heating too by increasing the power draw. Salt from perspiration is also something good to avoid with electrical devices and electronics.
