How to fix dc 12 led lights that burn out due to excess voltage or current
Unfortunately you do not tell us how much current is needed by your LEDs and fans.

Assuming that this current is less than 2 A you could consider using a buck-boost converter like this:



In the description this model is listed as usable up to 3 A but you have to take that with a grain of salt. I'd use it up to 1 A and not be worried. Maybe it would survive 2 A just fine, At 3 A it might get too hot and not work in the long run. Then it would be better to use a module rated for 5 A.

Actually the name is wrong as the converter first boosts the voltage up to for example 24 V and then uses a DCDC down converter (a buck converter) to lower the voltage to the voltage you need, for example 12 V.

This means the module will output 12 V as long as the input voltage stays within (quite broad) limits. So even at 10.5 V or 15 V at the input, you will always get 12 V at the output.

There are many different modules similar to this one, just search for "Buck boost module".