Two MP1584 voltage converters in parallel
The principle of all led circuits is based on current limiters. Usually a resistor.
In your case several step-down constant current supplies, in parallel, one for each led or series of leds, will be more energy efficient than resistors. Especially since your supply voltage is about double that of the leds. And it will give a constent output no matter the voltage of the dynamo which flucuates.
EDITO: I looked at the size of the step down board and I think they can be a very good solution given their compact size. They will stop working if voltage falls under 4.5V.
(how they can sell them that cheap is a mystery: Bellow the cost of components)
But it will complicate you circuit too much for such low power lights. Resistors will be much smaller. You can use SMD resistors on prototype board. And much more robust against shocks and weather hazards.
Search for LED circuits, resistor led circuits, led resistor power dissipation to know which resistors to chose.
1W resistor should be safe. Resistance depends on supply voltage and the desired number of amperes.
Make your choice.