I know you mentioned no higher than a 600, however I would look at a used Honda CBR650F. Comfortable, plenty of power for a starter and reliable.I see a 2004 Suzuki Katana 600 with 6800 miles for sale for $2k. They say it needs work. I might reach out and get it
Any thoughts on if that's a decent bike to start on and I won't outgrow? I'm sure it's better than a 300 or 400. And it sounds good going by videos