AWS Aurora Mysql different instance types for writer/reader
All the writes to a Primary are sent to each Replica and then applied there.  For this reason, the Replica should (in general) be at least as beefy as the Primary.
Often, you need to do reads on the Primary in support of what you then write.

Hence, the characteristics of the Primaries and Replicas are similar.
If the database is sort of &quot;write once and read many times&quot;, then you might want the Replicas to be beefier in general, but still a &quot;database&quot; type, as categorized by the cloud provider.
You may find that db.r5.large is overkill for your Primary, especially if you move a lot of the read traffic off to at least 2 Replicas.  (Based on my first bullet item, having only one Replica is not very helpful.)
If you want a deeper review of your application, provide RAM size, SHOW GLOBAL STATUS and SHOW VARIABLES.