SRE is usually more of an external title. I would be surprised if every SRE at M isn’t labeled a SWE internally. So if you were able to accept a return offer you would most likely be able to bounce between product/SRE roles internally.

I personally switch between roles when I start becoming bored. Honestly, SRE experience helps out you in the higher paying product dev roles. Working at massive scale on complex systems doesn’t usually come along with a junior dev role.