As a solo-founder/developer of [1], this is what I have been doing for the past year or so.
1. I searched for basic monitoring solutions for actively monitoring the backend and finalized New-Relic. They provide a free plan that is good enough for most startups. I have added a bunch of graphs for system, infrastructure and application monitoring. It keeps me sane and well-informed before things go wrong.
2. On my Digital ocean droplets and database, I have set up Slack alerts that page me in case there is a spike. I have created a free slack workspace just for this and added a different alert ringtone so as to not get confused with other workspaces.
3.I use Freshping to monitor Uptime and again, if things go down, I get email and slack alerts within couple of minutes.
4.I have Rollbar agent running for log monitoring. I get an email alert when there is an exception or error.
5. If I am out for more than half a day, I take my laptop with me.
6. I keep my phone on. Always.
In the last one year, rarely things have gone down. I mean maybe a couple of times.
Things I do so I can sleep properly,
1. I do not deploy before heading out, or on Fridays or at bedtime.
2. My infrastructure has a lot of redundancy meaning, a larger instance than required to handle a spike in case I am unavailable.
3. Database usually breakdown, so have recently migrated to Digitalocean managed database.
Things I am planning to do,
1.Try out Monit to automate some of the tasks.
2. Write down a list of steps or a runbook in case things go wrong. It is easy to forget steps when the production system is down.
[1] https://blanq.io