RDS Postgres + MySQL low read iops

I'm performance testing some tables in both RDS Postgres and MySQL with the following configurations:

• Tested each on db.m6g.4xlarge and db.m6g.xlarge instance type
• 1024 GiB storage each
• Running a SELECT * FROM very_large_table, all tables tested are ~80gb

MySQL: For the db.m6g.4xlarge instance type, I was getting variable results on 2 similar tables (exact same schema + same row count + ~same size), one table could be read at >10k iops consistently and the other table lagged behind at <3k iops, this variability was very confusing. Since switching to the db.m6g.xlarge instance type, I've been consistently capped at 1.8k read iops and a max read throughput of 26Mbs. This article says that I should be getting up to 4750 Mbs but I'm getting no where near that.

Postgres: Testing on the db.m6g.4xlarge instance type, reading from a table would give me just over 3k iops, 140Mbs throughput. On a db.m6g.xlarge, Im getting at most 2k iops, 100Mbs throughput.

Is there a way to guarantee a set number of iops per SELECT * query? It feels like my databases aren't being utilized properly with such intensive workloads. I've tried setting up an RDS MySQL cluster with 10k provisioned iops but my read queries were still capping at ~2k iops.