It’s not that easy as you imagine it. For a start I would do the following in the renderhook:
<img alt="{{ .Text }}" {{ with .Title}} title="{{ . }}"{{ end }} class="lazy" data-src="{{ .Destination | safeURL }}" data-srcset="" data-sizes="" />
with data-srcset and data-sizes with the values you need. I am not sure if you are working with https://github.com/aFarkas/lazysizes or any other script, but you obviously leave out a lot of what is required to make this work.
I hope you are aware that this render hook will take over ALL markdown images ().
Have a look at shortcodes, this might be more what you want.
And have a look at how to request help: