Electronic – LM317-based step-up boost voltage converter – what

You mention that you want fast switching. None of the devices will be able to do that. Any the feedback circuits internally are designed to be slow so that they don't to much incorporate noise. And in fact some of the feedback is thermal feedback. Put another way, the dominant pole is low, 100's of KHz is typically the the fastest response time. This is true whether you are using a disable pin or connecting/opening the load to the source.

The best way to fix this is that you build an external circuit that current steers between two limbs. On one limb will be your LED chain and on the other a dummy load that draws the same amount of current (and hopefully also has similar load characteristics). An example of such a circuit is a differential pair. You should be able to get 10's of ns switching time then.

The challenge with the two limbs will be that the load characteristic differences may disturb the feedback of the LM317 etc. which then reacts slowly. If you can't match the two limbs then I'd suggest building a current mirror that decouples the current output the LM317 etc. so that it only sees a constant load the mirror transistor sees the variability.

You should be able to simulate all of this in LTSPice or similar before building.

The other factors you mention above not show stoppers, so pick what you want.

Ofcourse the best solution would be to find the right IC but if you want the easy way then yes, you can use diodes to drop voltage and additionaly you could use series PTC and zener to ground for protection against overvoltage. Common values for zener are 33 or 36. There are hundreds of bucks with input rated >= 40V cheaper then 1$ so I`d go for it, especially as your fan does not take so much power to use LM2596 sorts which are more expensive and 2-3A+.