Hey, are you pouring the hot milk into the eggs too fast? Honestly, that always makes it lumpy. Tbh, you need to whisk the eggs really well and add the milk bit by bit. Also, mix your cornstarch with a little cold milk first to avoid clumps.
Adding the milk bit by bit really saved me last time I made pastry cream. I whisked the eggs with sugar first until they were pale, then drizzled in the hot milk slowly while mixing. That way, everything stayed smooth and didn't scramble.