Probably depends on your stack a bit too, but I'm using AppSignal for monitoring my Rails applications.

AppSignal looks like but the price is a little steep for a small saas that is hosted on a single vps

The free tier of services like DataDog, NewRelic, Grafana Cloud, etc.

Datadog's free trial is really limited although i am a fan of Datadog. Probably because i use Datadog at work. On the other hand Datadog is really expensive for a side project.

Newrelic's free plan is nice. Didn't know about Grafana cloud. Looks really generous.

render.com / fly's log viewers work well enough