I want consolidated application logs running on AWS ECS with microservices architecture

I would definitely make sure that you collect all metrics on AWS CloudWatch. For non-custom metrics, these are very cheap and integrate everywhere. Whether you visualize these in CW or in Grafana, doesn't matter. I'd recommend to stick as much as possible to a single source of truth when it comes to metrics.

As for logs, with ECS + FireLens you can send your logs to CW Logs pretty much any destination (fluentbit powa!).

But considering that the ALB logs will go to S3, I'd recommend FireHose / DataStream to log things in the same bucket / different buckets, same format. That way you can at least query the data with strong correlations with Athena for example.

With 1 FireHose per source account and a central storage location, you can also log things in multi-accounts without any issues pretty easily. We have done / are in the process of doing just that: monitoring account, aggregating / accessing all others to provide near real-time view of what's happening across all our services, with each applications teams considered to be their own tenants.