2x mullet drivetrain?

On my gravel bike I am running Shimano 105, with an FSA 46/30 crankset. The cassette is an 11-32 with a 105 short cage derailleur. Technically, I’m already exceeding the rear derailleur capacity by 2 teeth. When I’m fully loaded, I don’t really have a low enough gearing for my liking. Over the winter, I want to experiment with lower gearing. If I wanted to put an 11-40 or 11-42 cassette on the back, what are my most cost effective options?

I use a 105 50/34 crankset and a XT 11-40 cassette with an Ultegra RX medium cage rear mech. (All 11 speed). Works fine.