I don't know what this tool is or how it gets its "memory" metric. If you want to continue to use it, please ascertain that these values correspond to RSS by cross checking with i.e. ps aux. RSS is the memory exclusively held by a given process which is typically what mean by the "memory usage" of any given process. Note however that this does not count anonymous pages of a process that are swapped or shared with other processes.

Going into my task manager (Resources), I can see my using is using roughly 18/32GB of RAM despite closing all apps.

This does not tell you (or us for that matter) anything without defining what "using" means here. My system is "using" 77% of RAM right now but 45% of memory is available for use because it's cached.

Please post the output of free -h aswell as swapon.

Next, please post the contents of /proc/meminfo.

Do you use ZFS?

If you had something hog memory and a lot of other stuff got paged out of ram as a result, that can slow things down. Try running "top" and see how much swap space is in use. If it's more than a little bit, once you have enough memory available by shutting off whatever was hogging it, try "swapoff" (pages the stuff back in, which can take a little while) followed by "swapon" to re-enable swap.

Are you by chance using an integrated GPU?

Noticed that my AMD Radeon 680M uses quite a lot of RAM as shared memory.

Using something like amdgpu_top will show how much RAM your iGPU is using, metric is 'GTT'

Check if there's any large file in /tmp and /run/user/*?

Probably this but seems like a pain to use:

https://www.kernel.org/doc/html/latest/dev-tools/kmemleak.html

Do you use ZFS? It uses half your RAM for cache by default, which matches with 2GB used by user apps + 16GB = 18GB total.

does echo 3 | sudo touch /proc/sys/vm/drop_caches change the available RAM at all when this happens?

Stop using Newsflash or submit a bug report?