The short window of FO's going straight to Legacies is probably over for good in this hiring wave. The Covid pause, and the Boeing/Airbus/Pratt slowdown have given regionals time to staff up as senior FO's upgrade to make themselves more marketable when the hiring restarts. When the hiring restarts it will be the CA check airmen, then the CA line pilots going to legacies first. they "may" get to super senior FO's again near the end of the big 121 hiring wave (4 to 8 more years depending upon which carrier).... but don't think it will get to that again. We're on the backside of the huge hiring wave and the demand will decrease each year over the next 4-8 years. Then it will just be a normal attrition hiring schedule.

So, I'd plan on upgrading at either the ULCC or the Regional. If everything else is equal, I'd go to the ULCC. A junior Airbus CA will generally get street hired before a Junior RJ Captain. So, what's the upgrade time at the ULCC vs the regional? The regional jobs really aren't as bad as some say. The contracts at the better regionals have changed so much that they look more like the ULCC/LCC contracts from the late teens. So yeah, the wages and work rules are better at a LCC/ULCC but even the regionals are real jobs these days. Finally, where do you live? is the commute the same for the regional as for the ULCC, or do you live in base. I'd take living in base at the regional over commuting to the ULCC all day, every day. It makes it a completely different job. The quality of life improvement cannot be over stressed.

Don't go by who's getting hired right now. Don't let the fact that it's military and aviate mostly right now effect that decision without considerin the big picture. If things do NOT change then yes, go with the aviate program. However, if you believe the hiring will restart in earnest once the Boeing/Airbus/Pratt issues are resolved then they'll be back street hiring just like last time. If you haven't even started yet, your'e a good two years away from being hirable as they're going to go through all the Captains first, plus the military and the Aviate. Making a decision based upon who's being hired now is foolish, as the industry changes very rapidly. We all know the rapid crazy hiring is coming back once the slowdown is resolved.... absent a war or giant economic collapse it's a forgone conclusion.

Lots to consider when looking at the crystal ball. I'd do the ULCC unless you live in the regional base.