Well, let's see... Each tax return is a project. I suspect you could use a standard template as the starting point for each, then customize according to the particular client's circumstances. That gives you a project plan for each return.
I also suspect that you have a good idea how much time to allocate for each step in a more-or-less standard workflow for each return. Take the number of working hours until the deadline, divide by the number of hours needed per return (plus a safety margin), and you get the number of returns you can handle. That tells you whether you can accept new clients, whether you need to hire more people, etc. It also gives you a rough plan for what you need to accomplish in any given week.
So what about the work itself?
I suspect you'll find that much of the administrative work can be batch-processed: mailing returns, invoicing clients, etc. are obviously batchable tasks. So you might want to group all of these tasks together, either to do yourself at regular intervals or to delegate to an assistant.
For other kinds of work, you'll have to decide where the natural break points are and what things are batchable. That is, does it make sense to do all of the Schedule Cs for all of your self-employed clients, or to complete Client A's return, then Client B's, and so forth. Looking at those break points helps you decide what to do in any given interval of time. Those decisions also define how you'll want to set up your NA lists: what the contexts are, how granular you want to be, etc.
That information, in turn, tells you how to set up your tools. If it were my system, I would want to keep the surface view as clear as possible. That is, I would want a clearly defined list to tell me *which* returns are currently the most important, and what the next actions are for those returns only. And I would want to be able to look out a day, a week, or months in advance, depending. Outlook is pretty good at that kind of reporting. It's also used widely enough that there's a good chance at least some of the specialized accounting-oriented packages will interface with Outlook.
Beyond that, we're getting into accounting-specific issues that are outside my expertise. But I hope this helps you think about what the right system for you might look like.