I'm rather torn. I'm all for people undertaking their own assessments of systems rather than going to external contractors...but if you're looking for templates then that sets my spidey senses off.

Clearly I don't know you from Larry so could be a legionella guru but as its a peculiar system unless you can be confident that you have the necessary skillset across the engineering aspects for this bit of kit, microbiology and any relevant water chemistry then even though there is a good chance you could do it well based upon what you've said you are doing, there might be things that you miss. You're into HSG274 Part 3 in the main which is largely back to first principles. BS 8580:2019 will also assist you, possibly far more so than any of the other documents.

Not a template but some things to think about:

RO suggests you're reusing water - what's the source? What does the quality of the source water mean for legionella/bacteria growth? What would a failure in the RO membrane lead to in terms of contamination? How would you know it had failed? How effective is the atomisation? Vapourising water removes the LD risk since there is means of transmission of the legionella bacteria but is it 100%, and will it still be 100% in one years time? What temperature is water being held/stored at prior to RO? Is there a tank - they're often a bit naff and full of sediment/dead spots in my experience. Who is exposed to the water after release - what is their risk profile?