I tend to agree completely that there doesn't seem to be enough time (it would be years) left for Sam to forge a chain and return to Jon. Originally, Sam might have been at the Citadel for 4-5 years when we even learned that he had been sent there, but like for Arya and her training, the timeline change seems of little importance to Martin, which could be a hint he didn't plan for Sam to become a maester, at least not in the timeframe of the series, and that how far along he gets is not important.

I'm just not sure Martin means Sam to leave the Citadel before the end either. Martin now has him there for the internal (power reshuffling) and external (diplomatic, with King's Landing) aftermath of the Grand Maester's murder (while incidentally, Sam's dad is on the small council, and intending to deal with Aegon IRRC) if he wants to get into that (no doubt briefly, alongside a peek in Sam's research, setting other things up for the last book). That could be quite intentional, if originally he had planned to show Sam in Oldtown for the first time in the aftermath of Pyrcelle's murder and already a few years into his studies.

He also has Sam there if Marwyn ever returns or communicates.

There is his "connection" to Bran that could eventually play out (Bran could eventually speak through ravens), and eventually there's also the dragonglass candles. By the time this gets closer to the ending, there may be no real need for Sam to leave Oldtown to pass important information (to Jon, or whoever he needs to, because with their coup against Snow while they're surrounded by his wilding allies, the future of the current Night Watch forces is a bit uncertain).