2 priorities of seasoned legacy veterans: 1) schedules. 2) money. Upgrading generally falls further down the list for a variety of reasons.
Most could care less which seat they occupy so long as those 2 priorities are addressed. Live where you want, fly the schedules you want, make decent coin. It's why you'll find so many grizzled old 3-strippers at the legacies. FOs can make more in a wide body, with far better schedules, trips, lifestyle etc than a sweaty, stressed out narrow body captain flying 3-4 legs a day in crappy domestic routes. Some believe the captain sets the tone but at the majors and legacies it's not really the case. If there's an a$$ in the fight deck it sucks regardless of where you sit. Many cases of FOs telling captains where to go. It's a job, the romance is over, and the FAs don't laugh at captain jokes either.
If you go to allegiant, do it for what they have to offer. Great schedules, Good equipment, great bases. Upgrades are an elusive target which change dramatically so I would avoid that as a primary decision factor.
You are in a great place and time in this profession. You can pick where you want to go and stand a fairly good chance of getting there so understanding what will work best for you should be the first step.