Most RTOSs offer very similar functionality; learn one and it is easy to transfer the experience to another.
What is far more interesting is learning how to structure your design to ensure the operation is predictable, reliable, and easy to change as new requirements are uncovered. With hard realtime systems there are a whole class of problems (e.g. priority inversion, interrupts) that are completely unfamiliar to traditional software writers, but which will occur infrequently at the worst possible moment.
If you want to learn how to structure your thinking and design so as to avoid such problems, then take a look at the XMOS products. They have significant advantages such as a sound theoretical pedigree (CSP), cheap many core processors, FPGA-like i/o speeds, and a development environment that guarantees worst-case performance timing without executing the code. The starter kit costs only £12, and a pure software implementation can be a >10MHz frequency counter with 2 or more input channels.
Even if you don't use their products, you can use the design styles to improve your designs using conventional architectures.