Brake and turn signal indicators for ebike

I'm trying to control a brake light circuit made up of 2-6 led lights (depending on whether or not the trailer is hooked up) the LED's will be ran in parallel. I'm not sure what the resistance of the lights are yet, but here's the Amazon link:
https://www.amazon.com/dp/B01N5PWDHG/ref=cm_sw_r_cp_apa_i_Zu.qCb7PCG16G

The battery I'm powering the circuit with is 36v/500w. I'm using the following buck converter to drop the power to 12v:
https://www.amazon.com/dp/B071LGTTRN/ref=cm_sw_r_cp_apa_i_4lmrCbJY60K1N

I want to control the circuit using force sensor switch, but I'm pretty sure the switch can't handle the current so I need to use a relay. Here's a link to the switch:
https://www.amazon.com/dp/B004AO6AIS/ref=cm_sw_r_cp_apa_i_.U.qCbBCFK0VB

My questions is...I have so many questions. Mainly, can I just divert a little power after the buck converter to power the low voltage side of the relay? Here's a relay that I think will work.
https://www.amazon.com/dp/B00MMW0XWY/ref=cm_sw_r_cp_apa_i_ErmrCbCMJE3B9

It says the relay can be triggered at 5mA. But it also says the coil voltage is 12 volts. Does that mean I need to use a 12v/5mA current to trigger the relay? If so, what's the best way to step down the current to get that low of amperage at 12v? Or will the control side of the relay simply draw the 5mA's it needs so I don't need to step down the amperage to protect the switch?

I will also be powering 2 turn signal indicator circuits and one running light circuit besides, the brake light circuit, not sure if that's relevant. I was just going to run 5 leads off the buck converter.

 

Welcome to the forums.

This will take a few posts to resolve but right off the bat you can change regulators. The one you linked to is much more than you need. Go with a basic 36vdc--->12vdc convertor like in the link below. This one will handle 10A which is sufficient for all your loads.
36v to 12v

 

well, this may be interesting.....starting with the first part of your post...I'm not totally sure what you are saying with the 2-6 lights but assuming you want two light circuits in parallel?? As for the resistance of the lights, the LEDs are light emitting DIODES and as long as they have sufficient voltage they will work.
Battery seems fine with the voltage knock down...your LEDs are rated at 60 lms so that should put you around 1 watt of power per light...total current (amps) should be well within your 10 amp relay rating.
Your switch is a definite purpose device...made to do a specific thing, and I see no ratings on it. If you want to trigger your relay with it, it will have have to provide an input to your relay that is capable of + - 12v and 5ma.(and adjust your jumper accordingly) The input circuit should limit the current to 5ma...it is just saying that it needs that much from your trigger device.
Not sure what you mean by the "low voltage" side of the relay....can you explain?
If you intend to run more circuits off the converter, you have to consider the total wattage(draw) of everything that will be powered at the same time.
I hope that is of help, and I will need a better understanding if we go forward.