Find your hourly rate at your employed gig, and... double it.
More advanced work, niche work, for profit; no benefits, etc, etc.
Honestly I have no idea; the usual markup vs an employment is ~50% but it sounds like you're doing more on the side gig.
If you know and like the people you're side-gigging for.. just ask them. What do you think is reasonable for my time? Bearing in mind x, y, z I was thinking ballpark a to b..
Don't get too greedy, but don't undersell. How much 'arsing about' time is there that is NOT billable? How much travelling, dressing up in a suit with shiny shoes, etc?