Fluent Bit is a light-weight log processor and forwarder, and can be used as a replacement for Fluentd for these two features. Fluent Bit, however, lacks support for log aggregation and supports fewer input and output plugins.
We can either replace Fluentd with Fluent Bit, or combine the two, where Fluent Bit is deployed as a DaemonSet collecting logs from each node and forwarding them to Fluentd (deployed as a Deployment) for aggregation and routing to output destinations.