Thoughts as I run a similar (but completely different) setup on DO.

1) Have you considered DNS round robin for your nginx and varnish servers? Running 2 of each (at half size) would let either one die and nothing bad happen to your site.

2) Are you ok with MongoDB being a SPOF?

3) Are you okay with your main DB being a SPOF?

I ask this not to be snarky, but I see that you are 84% of the way to having a SPOF proof setup, but not quite all the way there. I know I sleep a little better at night knowing that if my nginx box blows up, I have more with DNS round robin to keep everything running till the morning.

Nice diagram, I'm borrowing it as a reference for the setup I'm going to be designing for work soon :-)

Did you consider running nginx for your PHP application servers as well?

Also, can I ask what sort of traffic you get per {{ time period }} ??

I would remove varnish and use nginx anyday. Varnish config makes baby jesus cry.

Like others have said, I'd ditch Apache/Varnish for Nginx/PHP-FPM with the ngx_cache_purge module. That's what I've done, and it's far easier to maintain. The VCL language has always struck me as overly complicated, but that's just me. Any reason you chose MongoDB over Redis? (I work with MongoDB and Redis on a daily basis, so I was just curious as to why you chose one over the other for session storage).

That's a great diagram. What do you use to create your diagrams?

Would it be possible to take something like that UML, package it up in Docker and have it deploy to your vps in a more complex heroku way? I think that would solve a few common problems.

Why do you dislike NGINX?