Often it's the "knuckle" of the derailer link that hits the seat tube when the derailer retracts fully, which then limits how short that the bottom bracket spindle can be in some instances.

I've used sandpaper to make a slight relief to the area of interference, with the grit side of course facing away from the frame tube and with the sandpaper moving against the derailer knuckle, but have also seen derailers fail because of too much metal removed from near to where the spring is seated!

A narrower chain always buys a little extra clearance within the front derailer cage, for what that's worth, so I often use 9sp chain even on older bikes with Asian freewheels with as few as 5 speeds.

Can you identify the exact point of contact that is limiting the front derailer's inward throw or retraction? Sometimes a little bit of grinding with a Dremel can increase inward travel, but again one can cause parts to break if this isn't carefully applied.

If yours is a modern "2-piece" crankset, perhaps the driveside spacing can be increased slightly at the bb spindle or between the bb cup and the frame.