Used to be you had to do 5 years in boats before you were able to request a transfer to targets.
That was when everyone, in theory at least, joined as GS. I volunteered during WEMs course and went from Collingwood to Dolphin so never joined a ship, but had the right to 'return' to GS as that's what I'd technically joined as. Now they're joining as SM so can't return to something they never were.
For the OP;
Runs ashore are great - and I've had plenty as a submariner. Of course I've never been on an SSBN though, and I'm sure for the sea time I've done I'd have seen more foreign ports if I was GS than I have. But then I'd have stayed onboard and had to turn to every day too, rather than stay in a hotel and slept off my hangover by the side of the pool. Swings & Roundabouts as they say.
But if you're looking at a career in the RN rather than using it as a way of moving your genitals from port to port, then consider the bigger picture - what would you be happier doing, GS or SM? You can, I'm sure, ask to change from SM to GS before you join, but whether that'll mean a longer wait or if they'll make you start the application again I don't know.
You can ask to transfer in-service too - and you might be allowed. But it's not a right and they might well say no, and there's nothing you can do if they do.
Consider carefully. The decision is yours to take because you have to live with the outcome, but I'd warn against making a quick decision for what could be seen as a frivolous reason.
