Quite a lot of research has been done on task planning. Since this is a Stackoverflow, not a business site, I will compare it with the kernels of the operating system;)
You have several tasks that need to be completed, and the goal, of course, must be effective. Efficiency can be measured as high throughput or low latency.
When switching between tasks there will be a costly penalty called context switching. That is, you will have to switch documents, applications, enter the system, etc., and it will take time to get into the zone.
Given several tasks of different sizes. If you randomly arrange them in a sequence (batch), you will have the least number of context switches and, therefore, the highest overall throughput. If you additionally select them in size, from small to large, you will also reduce the delay. Since the client with the smallest projects will be delivered first, you will have more projects that will be implemented at a certain point in time than in the case of the random approach above. The problem, of course, is what to do with large projects when new smaller ones arrive. This can be solved by applying some timestamp. If some projects are more important than others, you will have to attach them accordingly.
If this still does not give you a satisfactory delay, you will have to multitask. This will reduce throughput due to expensive context switches, but you can satisfy your customers by showing them that you are making progress in your projects. The important part is to choose how to allocate time intervals. Per hour, per day, per week? The shorter the time interval, the more context will switch.
Tasks, in turn, can be divided into two different types: tasks with CPU and I / O. I / O interaction can be considered as tasks requiring input from a device / someone else, while binding to a processor requires only your time / capabilities. Organize your week to get started with I / O tasks sooner rather than later. While you wait for their return, you will focus on tasks related to the CPU.
Summarizing. You break down each project into smaller deliveries that are small enough to meet your customers' progress needs. On Monday morning, you plan your week and start with all tasks that require the actions of someone else (statements, etc.). Send these emails and make these phone calls. When this is done, you will focus on the parts that require your concentration.
Or you could hire more people :)