757 is an option for new joiners in theory, but not the right option for you in my opinion.
By your comments you would join with less than 2000 factored hours - the money here is poor as an FO until you get your third stripe.
Flying less than 500 hours a year means you are at least 3 years away from even applying for command, at which point the 757 fleet will be at least 30% smaller, likely only 1 or 2 years left by then, with everyone who is already here ahead of you in the que for the LHS.
Not to mention the training department are not going to entertain running a full type rating course for a TR 737 FO when they can just put them on an OCC.
Direct entry captain is a different story, hold your nerve and you may get your fleet of choice.
Overall the company is in a good position and a good place to be.