Cost Management for RDS db instance

I had bought a reserved instance for RDS db instance db.r5.4xlarge on January-05-2022 for 3 years duration. In the month of December-2022, I had to upgrade that instance to db.r5.8xlarge because we were facing some high database load. Since then I saw the increase in billing and I need your help to bring down these additional charges.

I'm expecting to save the overall cost by $1500.

I see that the reserved instance db.r5.4xlarge is being 100% utilized, at the same time $320.14 got charged in this month for USD 0.4303 hourly fee per Aurora MySQL, db.r5.4xl instance which is not there anymore in my account. so this $320.14 charges are extra for me.

Then, these charges USD 0.021 per GB-month of backup storage exceeding free allocation for Aurora MySQL costed me $283.38 so far.

Kindly advise how I can reduce the cost?