No way on expressjet, they are losing 100 odd aircraft this year, not sure how they figure upgrade will drop unless attrition is insane.
Envoy flow is nice but takes a long time to flow and the flow will slow if staffing becomes a issue. Their credit per trip is awful, upgrade time is a long time, but I'd take flow over guaranteed interview.
Endeavor I don't know much about their current situation but I always question why someone needs to pay huge bonuses. No business owner does that out of the goodness of their hearts.
SkyWest is solid, good pay, great bonuses (which have been around for forever minus the two pilot profit sharing ones), lots of bases, High quality of life. Don't expected bonuses (besides type over 12,500 / 121 or 135 operator bonus of $7500). Ran into CEO the other day and he said no way on retention bonuses at the moment, no point paying for people to stay because they will leave first chance they get, you only stop a few from going to LCC and very few leave with upgrades so low and mainline hiring being what it is.
Comes down to where you want to live and if u have anything that can get you a mainline interview faster than flow or if you have something in your background that flow is essential.