Astable oscillator with voltage dividers as source voltage?
"which will eventually become an SMPS"


Unless you are thinking of low voltage and low current, DONT TRY TO BUILD YOUR OWN SMPS. 


"If I only supplied one voltage divider, every time a transistor turned on all the current would be sucked to ground."


I can't imagine what you are talking about, which even more convinces me that you should not be building an SMPS.


The only difference I can see here is the bottom resistors forming the voltage dividers.


A difference I see is that you use 1nF instead of 100 nF capacitors, so you frequency would be 100 * higher, which might be a problem.

A big difference is that you seem to want to replace the pull-up resistors by equivalent voltage dividers, presumably because you have a higher power supply than 9V? (You probably don't need to bother, those 2N3904's will wrok well up to much higher voltages.). But you screwed up your calculations: take the leftmost resistors (R1, R2): 500/700 in parallel will act a s a resistor to 9V if you power is 1200/500*9v = 22V, but the equivalent series resistance is ~ 390 Ohm. That probably does not matter much for those resistors, but for R3/R4 it is a big problem.

IME the circuit you show works well without R2/R5 and the diodes. In most cases larger values are choosen for R3/R5, with correspondingly lower capacitor values.

PS I think you approach of taking a know-working circuit and repurposing it for your own projects is fine and can be educational, but you should select a project that is reasonably free feasible and free of danger, and you your theoreticalk knowledge (in this case, calculating the Thevenin equivalent of a voltage divider) should match the level of thinkering you want to do.