decreasing CPU number of cores on DB server

I have a RAC database 19c 2 nodes on Linux 7 with DB 19.3, i have allocated 8 cpus on each node, now the database has been online for 6 months in which i was monitoring the CPU Ram usages on the server ive noticed the below:

• CPU is almost most of the time with 90% idle (evern in peak hours), except when RMAN backup is running , i see a peak on CPU during the RMAN backup that reach almost 100%. is it safe to decrease the cpu number to 4 for example with no effect on the DB knowing that its almost 90% idle most of time, if yes what would happen to the RMAN job would it fail for low CPU or it will accomudate the change and it will use the current max cpus, and would i expect any performance degradation on the