At what stage should modest/realistic professionals consider themselves a true “subject matter expert” on a topic?
Stage never
It’s all relative. Stephen Hawkings is a physics expert from my perspective. If he became twice as good at physics, he would still be an expert and I probably would not even comprehend the difference. Similarly, if you have modest knowledge and talk to someone with zero knowledge, you are an expert. However, if you talk to a peer with equivalent knowledge, you are not an expert.