Metal pads wear longer, and have a stronger initial bite, work better in the wet, but modulate worse. If you like your current braking resin is fine, and is probably what I'd prefer on a road bike.

Shimano mineral fluid turns gray in the master cylinder because the seals absorb the dye, and this isn't necessarily a sign of serious contamination. When it's seriously contaminated it turns closer to dark purple/black.

Your problem doesn't seem normal to me. The most common cause of caliper failure on Shimano brakes is cracking the ceramic brake piston, which can be caused by forcing the caliper in at an angle or simply overaggressive tool use on the pistons. If you have time on your hand, you could try removing the caliper, disassembling it, and pushing out the pistons and inspecting them closely and reinstalling them. Otherwise, yeah, new caliper.