This "serious reader" expression makes my skin crawl.
Like if it was something of a sport with olympics where people compete in their own weight and it is measured in the end to the hundredths of seconds in front of spectators in a stadion shaped library cheering READ, READ, READ! Quality is mentioned, remotely, through selection, but still, the mental picture remains the same. The post smells like a training guide from a large gym franchise for readers. It's name is 'Serious Readers!'
I don't think serious reader is a cringe term. Reading is a skill like any other and we use terms like "serious golfer" or "serious painter". The flip side of serious is causal. It's okay to structure your life so that reading is a big part of it.