Sorry there are some things here that just don’t make sense.
I don’t think it does. Snaps are still on 3.15.x You are running snapd, but that doesn’t mean a snap is installed (and to save some memory I’d disable it if you are not using it as it is a resource hog). I think DO use the rocketchatctl script and it does a manual install. Hence the version.
Note there is a lot of difference between 1Gb and 2Gb at this level. You can’t easily compare them. We have already told you that the OS will take around 1Gb of RAM just to run these days. Yes, you can get leaner systems than say Ubuntu, but for your typical installs that’s 2021 for you.
So you really need a min of 2Gb to run Rocket comfortably. 1Gb will almost certainly give you issues, as you have discovered.
This is not what it appears…
The kernel dynamically uses RAM for buffering & caching to speed up operations and will usually grab as much as it can, but will relinquish some of that if an application needs it directly. So it is not unusual for you to see almost all the RAM used no matter how much you have installed - you can see Freemem is 166.7M in your screenshot and the Cache is 923.4M - that’s nearly 1Gb the kernel has reserved for itself!!
As the applications use RAM the kernel has to reduce the buffers & cache. But it has a minimum requirement for itself. Once you start squeezing this you are in a world of pain. For Ubuntu 20 they recommend a minimum of 512Mb RAM just to run. That does not include ANY services. Just the OS.
On your 1Gb droplet the applications - both Rocket AND Mongo - will want RAM to operate. Check Mongo requirements for the application, and caching - it needs quite a lot. Probably both need more than the 512Mb left. And you have no swap. And almost no buffers or cache…
You can of course help a bit by using a swap file but it is just a slow alternative to real RAM.
I think you’ll find on your 2Gb droplet things will run without any issues. Let us know.