The Performer will give you a LOT of options, including modulation similar to the internal sequencer in the classic Native Instrument's Massive VST. There is also a lot in terms of Euclidean patterns, chance, etc.
You're going to get a module a month. Sounds like a plan. Take the time to really learn the Performer and the Assimilator. The Performer has a lot PERFORMANCE options, so learning them will really help you out. The Performer can also drive external MIDI instruments as well. So if you have any hardware MIDI instruments lying about, you can work with them as well.
My thought on your next module to purchase would be the Disting EX. You can link your F8r via I2C. So it would make sense to get the Disting since you have the F8r. The Disting will also give you a nice selection of different functionality that you can try out as you explore your modules.
As you gain more experience, you might change your mind on what modules you'd like. So I'd get the Disting and see how you feel a month or two after.