Or just host the grafana (or different) stack yourself.
You want free and easy and polished experience.
Grafana works fine. If you want something better than that, you'll have to pay for it or create it yourself.
Of course, it doesn't have the 293428492 features that Datadog has, but works well enough for the main APM use-case
Currently also offers free AI-powered anomaly detection and RCA, along with a full-scale Incident Management module.