Hi . .
Avada's really heavy because they use composer and prior at the script placement which not good for loading. I could usually test my speed in Google Insights which very reliable as speed and performance tester
I have been working in wordpress performance. I have done a lot of themes and sites which always prior performance. I could improve your wordpress without changing avada itself. There are a lot of features of avada instead changing it to another theme
What I would do is front end optimization. I would move, compress the assets of site like image, css, and javascript. And do server optimization ( caching, page encode ) in order make website more faster
As for registration and other requirements, I would prefer to use existing and strong plugin. With a custom and a bit of modification, it would meet your requirements for sure :)
Please take a moment to visit my profile page, there are reviews and portfolio could be considered here
Thank you and
Regards,
Shelingholmes