I'm looking for opinions about what LightSail instance would be acceptable for a low-traffic LimeSurvey deployment (less than 500 visits per day). I'd install DirectAdmin to provide a simple way to manage the single account that would be on the LightSail Instance.Is 4 GB Memory, 2 vCPUs offering suitable for this deployment?

No answer to the direct question, but as LS 5 is EOL, you might want to look into LS6.

And with LS7 in beta, even LS6 might be EOL sooner than later.