That’s what I originally thought judging by what you’ve said regarding the error noted.
As I said in my previous response, it is possible to run a working stack on 512MB, but it’s not quite as easy as set and forget. You have to constantly tweak and tune configuration for all services running (NGINX, MySQL/MariaDB, PHP-FPM, Redis/Memcached, etc).
From what I can see, you have ~30 plugins installed and active. That’s going to be taxing on limited resources.
The WP-Smush and EWWW plugins are redundant and taxing on the CPU as they both require CPU to handle smushing/compressing your images down.
The more images you upload and are processing, the more intensive. You don’t need them both, so I’d pick one or the other.
If you have Redis/Memcached installed, that’s going to chew up ~32-128MB of your RAM at any given time. The defaults are 64MB. Unless you need these services and are planning to run multiple servers, you’d be better off using PHP’s OpCode Caching. In most all cases, it’s going to be faster than Redis or Memcached as it’s handled by PHP natively and doesn’t need to be offloaded to a service.
Now, if you were running NGINX on one Droplet, PHP-FPM on another, your database on another, etc – (i.e. clustering) – then yeah, Redis or Memcached would be perfect.
I would recommend dropping Redis/Memcached and using PHP’s OpCode Caching for object cache. I would also choose either WP-Smush or EWWW, but not both. Additionally, reduce your plugins down to those you absolutely need – deactivate anything you aren’t using or don’t need.
Beyond those recommendations, it’s more of a challenge of tweaking and tuning, or upgrading your Droplet so that you have more RAM available. As noted above, I recommend 1GB minimum, but at least 2GB if you’re going to use this as a production Droplet that receives traffic so that you have the room you need to grow.