Every bit of this is true.
The attrition at some of the regionals is currently higher than it has ever been. The hiring at the legacies has set records in the last 2 months. There is more movement right now in the industry than any time in the last 20 years.
Major airlines are dropping their requirements to get enough candidates to fill classes. And it is only November. It really hasn't even started yet.
Right now, there is absolutely no way to tell which regional airline would be best for you in terms of upgrade and upward mobility. There are some really bad choices (avoid 50 seat airplanes for United, for instance), but no one has a crystal ball. You could choose somewhere with what appears to have the fastest training and upgrades, and they could completely fall apart operationally in the next 2 months and upend everything. All it takes is a handful of key pilots to leave (check airmen, managers, training department) and everything can get delayed.
One thing is for sure, if you are already at a regional, make sure your application is in and update it at least every other week. Even as a new hire FO, get that app in.
Make yourself as "attractive" as possible to the majors and be ready to go when you get your chance. With hundreds getting hired weekly, don't lose out on a LOT of seniority.