It’s difficult to give you advice on this bit without knowing you (and I think it’s something you’ll know best). What I would say is vacation schemes are great for experiencing what a lawyer is, how the people are and for getting a vibe on what the firm is like, but they don’t give a holistic view of the firm, not least because they’re extremely intense – my firm’s vacation scheme is renown for being extremely assessment heavy and the actual experience as a trainee is very different. You’re still trying to impress but you can have a lot of fun in the process. Stress-wise firms could do a lot more to manage mental health, though I wouldn’t underestimate your ability to manage the work especially if you’ve got this far. It’s a lot less difficult than it sounds and much more about project management/organisation than real technical ability.
I think it’s right that you’re thinking about the long term though, few candidates do. I’d suggest applying to all of them. Normally I recommend against a scatter-gun approach of applying to law firms, but in this case you know your stuff and the best way to learn what works for you is to try get on more schemes (or at least an open day) and eliminate by trial and error. I ended up turning down a few TC’s until I found the my current firm; it was risky but if I didn’t do that I don’t think I’d be enjoying myself as much right now. A final point to remember: it’s easy to move down once you train at a prestigious law firm, however it’s very hard to move up.