I think that summarizes what I suggested here Re^8: Perlmonks site has become far too slow (Reverse Proxies).

Using nginx reverse proxies with rules to only pass thru logged in users (or yet uncached requests) and alternatively serving static content to anonymous clients (or blocking them if needed).

The EE can flag updated content such that nginx would update it's cache automatically.¹

Looks like an out-of-the box solution for me which can easily scale.

Nginx would grow it's cache gradually and carry the bot load while keeping the logged in users operational.

¹) That's the only change we would need at the backend which can be implemented and improved gradually.