Level, I hate to stereotype, but I think the best work in a commercial firm for pt hours is property. Because you don't tend to just work on one or two transactions but a lot of files at one time, with paralegals to take the brunt of the day-to-day grunt.
Avoid hard core transactional work, like corporate takeovers, mergers, initial public offerings, and financing work around that as well as transactional loans. The feature of these transactions is that they tend to be big, take up almost all your time (and thus not easily delegable), have client-led timetables.
If possible, do advisory, rather than transactional work ie giving advice rather than drafting documents for a transaction. These tend to be slightly better. Although I can't really think of any field that is purely advisory, there tends to be a mix. However, I imagine that tax, employment, regulatory, would have a greater portion of advisory work.
I hope I am not talking out of my bottom. I am ex-capital markets at a City firm and so my understanding of other areas of law is sketchy at best!
But a lot of this depends on your firm and its clients. So use your training seats to get a better idea of where to go. Good luck. It is not easy to work pt as a fee-earner.
Though I see it working a lot more successfully in support roles, like PSLs.