Impact of adding new rules in Block mode

The AWS WAF is basically linux modsecurity so no matter if you use the F5 rules or AWS ones this are the AWS WAF limitations as there is no machine learning or new rules or modified being placed in learning mode  like the F5 signatures.

The only option I can suggest is to use AWS waf version managment and to change the "default" setting to a specific version and each week to manially check for a new version and if there is a new version to select it and then to test in a test window. you my try to automate this in some way.

https://docs.aws.amazon.com/waf/latest/developerguide/waf-managed-rule-groups-versioning.html