“Look, your PhD is probably the worst research you’ll ever do, because you’ll get better asdfadsf ”

Look, your PhD is probably the worst research you'll ever do, because you'll get better at it later. So just do it so you can move on. Pick a handy function you'd like to have and think, what's the hardest part, the keystone' that makes this possible? What's the keystone of the keystone? That's a high-leverage thesis topic.