I would visit BFold and see if they are willing to modify your M3L to a M6l for a reasonable price. $500 sounds steep for a relatively minor mod.

Bike Fridays are great bikes. We checked out the new Silk model in Eugene, OR. It is available with belt drive, Shimano Alfine 11 speed hub, and disc brakes with various tire options. 20" Bike Fridays feel to us just like our 700c bikes. But their folds are nowhere as elegant or compact as those on a Brompton. If I lived in a 4th floor walk up I would stick with Brompton.