From my experience, that sort of simplistic MPPT controller is not very good and a 555 will be way better.
At 20W and low voltage, it would probably make sense to build your own solar panel. Go for 10 or 11 cells in series (or 12 if you want the option of using conventional lithium cells later on), and then a LDO regulator with reverse current blocking set to about 3.6V would be all you need for a charge controller.
Go higher power, then just buying a premade panel would make more sense.