I'd like to stay with Redis - it's a battle-tested, hugely scalable and ridiculously performant key-value storage. No matter what happens in the future, Redis will grow with it.
The responsiveness is perfectly fine, we've got four cores dedicated to nginx and NodeBB, that works well. But I think it would be a nice addition to NodeBB to be able to not hold less frequently accessed content in memory but load it on demand. I'd imagine that'll be also interesting for huge-scale instances.
I'm currently using DigitalOcean, yes, because I like VPS servers. I'm a fulltime sysadmin, many of my work clients have their own datacenter servers, so I'm aware there are cheaper possibilites. Though I think the huge advantage of a virtual box is the speed to scale it, and the direct control over its power supply and console.
I've just looked at SSDB, but I wonder what the main advantage is? The stats seem to indicate it is a little bit slower than Redis, but funnily enough the page doesn't list any selling points... Less memory consumption?
I could clean the database up, but to be honest, while I'm the admin and responsible for the server, I'm not really involved with the content or its users.
For the stats, here you go:
Keys in Redis: 2,402,609
Page views per week: 549,187
Uploaded files: ~4GB