It isn’t too late to start. Actually, it’s maybe a good time to start, as a lot of people have gold to purchase lower level items you may want to put up on the AH to help fund your leveling gear, mount fund, etc.
I’d recommend looking for a leveling guild somewhere. Some of the servers have pretty long queue times at the moment, particularly during peak hours, and of those, some currently are restricted for character creation, so obviously those are off the table.
We have some data available for roughly gauging realm populations, and faction balance (if that’s something of concern to you):
https://wowclassicpopulation.com/ and https://ironforge.pro/servers/
Whichever realm you end up choosing, I hope you enjoy your Classic experience. I certainly have, so far! 