How to down size the storage of AWS RDS MySQL instance?

I have an RDS MySQL instance db.m5.4xlarge with 8.3TB of data, and I want to shrink it to 4TB to save costs because it actually has 4TB of data. What are the viable options? I've tried several methods, aiming to complete it within 8 hours: 1. DMS (Database Migration Service): When using DMS, there were index issues. Before the migration, adding indexes caused it to report duplicate IDs, but I checked and couldn't find any duplicates. After the migration, if I add the indexes back, the data doesn't sync. 2. Mysqldump: It takes too long. For 450GB, it's already taking nearly 6 hours. 3. Take Snapshot and restore to smaller storage: but the options on AWS console restrict me to downsize to a volume that is smaller than its original size[enter image description here](https://i.sstatic.net/kEyNI6ib.png)