With multiple types and thousands of hours of jet 121, your only hangup is currency/recency (and as you say ATP for most majors).
Currency is easy, for recency you'll need to check out major websites to see what they're looking for and also probably check out the forums to see what's really happening in the trenches. I don't know if CFI or part 91 ASEL GA recency will get you called by a legacy. In these crazy times it's certainly possible but you'll want to try to get some intel on that. You might need some recent turbine or piston 135 time.
Regionals would have you as a DEC in a heartbeat, and pay $200K-ish plus bonuses (you'll get pay longevity credit for typically ten years of your prior 121 experience). They'll also give you the ATP training in the process. DEC is pretty sporting if you're not current, whether that's a good idea depends on your self-assessment of your own flying/training ability. If you did the DEC process you could then apply to majors immediately with wet ink on the ATP, although legacies are metering hires from their own regionals. You could go back to an AA regional, and then apply to other legacies though, they're more than happy to poach from each other's regionals.
Figure out how much and what kind of recency you need for legacies, then decide if you're better off chasing that on your own (and paying for ATP), vs. going to a regional.