There are lots of us here who have gone from PPL theory to ATPL theory; some would have found it easier than others. If you are good at maths and physics and enjoy learning, you should find it OK. If you are struggling with PPL, then you will likely find ATPL harder.
I found it quite straightfoward, some subjects really easy, others not so but the volume of information to learn is quite high.
Cheers
Whirls