Ships deploying on HR are overfull with people; you usually are fighting for each department to get spots. If you have trainees they are already qualified at one job and training for the next, or otherwise bringing something to the table (like speaking a specific language). Ships are really complicated, we reduced crew numbers by giving everyone multiple roles, so it takes time to get someone up to a baseline of being useful.
With ships sailing on skeleton crews you don't have enough people to do any supervising and actually train people. MSE departments are rolling with around 25 people total out of the 50-60 they are supposed to have, so while you may have room for another 100 people, unless 25-30 of them are MS-PO (with a few officers), there just isn't enough people to run an entire department worth of S3s through some useful training, handle the admin, or keep them out of danger in an emergency. Having an extra set of hands to help a trained person do a task can be great, having 4-5 untrained people per qualified person is just a huge burden, especially when they already have more work than time.
We've also been pretty good at pulling instructors out of the schools to fill key positions to get ships at sea, but facilities, equipment and experienced teachers is all part of the logjam. We shut down the programs we had to run people through college programs and then provide a bit of delta training/OJT, so those would have to be stood back up again (which also requires a few experienced people to help run/administrate), so even our external surge options are gone.
Things like the DC school also have limited throughput, and those buildings are 25 years old and need some TLC, so that would probably be the most obvious chokepoint for getting people through their initial training before they are on a ship, but even after they get basic fire/flood training, really nowhere for a big chunk of sailors to go.
Our input to the training system has dropped a lot over the years, so the infrastructure has scaled back accordingly. The last SIP I saw from 2019 had something like 120 people recruited that entire year, with comparable numbers before that, and people still spend time on PAT waiting for courses, but gives you an idea of what the system is set up for.
Fixing it is all doable, but some of these things would take a few years to get setup if we start now, IF the RCN takes some significant steps to slow down the OPSCHED, maybe retire some ships, and park others so we could actually have enough instructors, while doing things like night courses or shift training around the limited training spots for the equipment specific stations.