But a sand bag add on armor set or the deep wading trucking used on D Day might be a lower piece count add on
Rubicon's M3A3 is a late version - they didn't come ashore on D-Day. A sandbar armour set would be nice, but would need a lot of parts to do properly.
My vote would be for the Jumbo. That would require a new turret, additional armour for the hull sides and glacis, and tracks with extended end connectors (all Jumbos had them). Again, quite a lot of parts.
The issues with the current M3A3 could be fixed by replacing the turret bustle underside piece, making the bustle more rounder, replacing the mantlet with a larger one, and re-doing the cupola (mostly the hatch). If you were going all the way, you'd also fix the exhaust vent and provide a chunkier .50 cal.
Approaching it from another direction, what if you provided optional parts to build a late M4A2? They were used by the Marines and the Soviets. It would only require an alternative hull (or, if you re-did the hull with a separate engine deck, optional engine decks), and optional rear hulls. Everything else (including the 76mm turret) would be usable for both the M4A2 and the M4A3.