Morning Alex,
Yes, nginx was something that I considered and it looks like a similar product to modsec, but nginx appears to have a few more bells and whistles. I do not know this product (nginx) to be honest.
Forums are not about being perfectly correct but about helping others with constructive ideas and suggestions. Forums help people to be better people by helping others when they have questions but unfortunately my post was read by a large number of people and ignored before you answered. And still ignored :-)
We all go through a learning curve and stumble at a hurdle, its how the people around you help you with that hurdle.
Thanks!
For reading (I know its the other way round but its worth a read)
https://www.nginx.com/blog/when-how-to-migrate-your-f5-big-ip-load-balancer/