Why would you ever quit being a G550 captain to be a regional FO? I think a move like that, if the goal is to be more competitive for a legacy, would raise many red flags to recruiters. They will wonder, was this applicant 'forced' to resign, etc. If you haven't gotten a call from your carrier of choice it's unlikely that its due to a lack of flight experience. If you are fortunate enough to be hired by any LCC/legacy, I'd take the job and keep working toward whatever your goal company is.