While Grafana Agent uses less resources than Prometheus, there is more optimized Prometheus-compatible scraper and router exists - vmagent [1]. I'd recommend you giving Grafana Agent and vmagent the same workload and comparing their resource usage.

P.S. Prometheus itself can also act as a lightweight agent, which collects metrics and forwards them to the configured remote storage [2].

[1] https://docs.victoriametrics.com/vmagent/

[2] https://prometheus.io/blog/2021/11/16/agent/