Can I make this filling as a custard by itself on the stovetop? Then cool and refrigerate to eat without the crust?

Sure, you can bake them in ramekins, but if you do, I’d bake them in a water bath (bain marie).

Put the ramekins in a large roasting pan. Then fill them not quite to the top with the custard. A pitcher or large measuring cup makes this easier. Put the roasting pan in the oven and then, using a pitcher, fill the pan with water to about halfway up the sides of the ramekins. Bake them at 350 for around 40 minutes until the edges are set but the middle is still wobbly, but not wet-looking. They’ll continue to cook and set up when they’re cooling.

Good luck!