Can I say I admire your inventiveness?  You go to some lengths to not register and really, apart from the majority of people not knowing how to do a reverse image search, probably reflects people's attitude to those sorts of lightbox registration forms.

Heya there,

Thanks for asking your question here

My first point would be that human visitors don't like to be given forms when they first visit a site, so would suggest you don't do this.

My alternative strategy would be to provide a home page of good content talking about the data etc that is available on your site and then provide a button for people to register if they want to.

Don't detect the user agent and provide alternative content as, however good your intentions are, that could be considered cloaking. Google is against you providing Google different content to humans, so don't do it.

Do things differently