Having had some of this fun in a previous life with Wordpress sites.
I found https://www.wordfence.com/ super useful in finding all the files in the sites that had been compromised. Once you are compromised you can find the attackers have left several ways going forward for you to be compromised again. I would run this, they will verify your installations and files are not compromised still.
To verify you got your security in line in AWS run these excellent tools linked below, they can be a little overwhelming at first but worth the investment if you going to run long term in AWS.
Scout suite
https://github.com/nccgroup/ScoutSuite
Prowler
https://github.com/toniblyx/prowler
If you are new to managing word press sites in AWS then you cannot go far wrong with something like Bitnami Wordpress images https://bitnami.com/stack/wordpress/virtual-machine. These are out of the box setups and reasonable good setup and practices for hosting from the get go. They do come with some draw backs we found with major upgrades but plenty of documentations on managing that.
Check your logs also is good way to potentially spot what is compromising you and give you some google paths to go down.
If you want to carry on adding layers take a look at WAF in AWS. (https://aws.amazon.com/waf/) That will give you some extra protection if you are still getting problems.
hths, I feel your pain.