The absence of any flour or custard powder means that the lemon curd can’t be boiled or you will end up with scrambled lemony eggs. To play safe, warm the lemon curd in a bowl on top of a pan with simmering water (au bain marie).
At 65°C, the egg starts to bind the curd. At 75°C, the egg are cooked enough to be safe to eat.
The less butter you add, the thinner the lemon curd will be.
Add dried cranberries to the lemon curd.
Use a different fruit like orange, lime, mango or raspberries.