Honestly, the time to go to Spirit was last decade. 2010-2019. Sure, there is growth but that can change in a heartbeat. Retirements are set in stone. Spirit has one of the youngest pilot groups with the least amount of retirements coming up. In their crazy growth spree, they had sub 3 year CAs for quite a long time. You will be stuck behind CAs who are in their 30s and 40s.
If I was a young guy starting out today, my sights would only be set on the big 3: AA, UA, or DL.
With the crazy hiring coming up, you should be able to get picked up at one of these three places. And with their MASSIVE retirements, you will always have a nice steady progression up the seniority list.
Flew with a FO recently who had his apps in at the big 3 and never heard anything so he let Airlineapps expire. He said for kicks, he just paid and re-updated everything this summer. In one month, he got cold calls (out of the blue) from DL and UA for an interview.
And I'm not picking against Southwest, ULCCs, LCCs, or smaller legacy airlines (HAL, AS). I myself went to a LCC in early 2012 but at the time, that made sense. NO one else was hiring. In 2011, the only hiring was Spirit, Virgin, and I think two windows opened up at US Airways that year. That was it. It made sense to go to Spirits and the Virgins back in those days.
Today? and for the next 10 yrs? Fuggetaboutit. Hit the big 3, hard.