Your schematic, the right 555 would generate a frequency of ~ 2kHz, with the left @30Hz. So the right 555 would switch between two frequencies at 30Hz, because the output is either high or low - it won't slowly move between the two frequencies. Also you don't need C4, it should be a low value resistor, say 100R. The reason is, often circuits with two 555's are generating a "siren" which drives a speaker - this requires a large value cap on the output to drive the speaker, but you're driving a MOSFET which is driving an LED.
And.. you're using a P channel MOSFET, on the high side, wired backwards. I would use a N channel MOSFET, on the low side (source to GND, drain to LED cathode) or an NPN bipolar transistor. I can provide a schematic if you don't know what I'm talking about.
Two things:
1) although you have explained your requirements quite succinctly, am I correct in saying you wish for the 8Hz strobe to slowly increase in frequency to 20Hz over the course of 15 seconds, then ramp down again over another 15 seconds? Or do you want to immediately go back to 8Hz again? The difference is, the first requires a triangle wave, that reaches its peak in 15 seconds, then back to its low in 15 seconds, and the latter requires a sawtooth wave. A sawtooth can be generated with a 555 and an extra transistor, but I am unsure if one can create a triangle wave with it. THere are opamp and comparator circuits that can do this.
2) Do you have access to simulation software? I believe LTspice is free, and includes an NE555 timer model. Its fairly accurate and you can play around with it, trying different ideas and values to see what kind of output you get.
You are correct in saying that its quite difficult to obtain a specific frequency when you're changing the voltage on the "CV" pin of the 555, as this changes the thresholds of the internal comparators, and so, changes the conditions - this means that the equations we use for the 555 are no longer valid. This is why I suggested using LTspice.
You don't have to use 555's. There are comparator and opamp circuits that can produce triangle or sawtooth waves, as well as square-wave oscillators. You may even requires a combination of the two, but its certainly "doable".