Why can't I use FORMAT in Fra?
One of the advantages of the FRA is that it is self-managing. IF you use the FRA, then when there is space pressure on it, any files that can be deleted will be deleted - automatically.  With the FRA, you don't need your powershell scripts to try to manage your backup storage.  In fact, you don't want your powershell script to manage your storage.  Let Uncle Oracle and rman do it for you.  You say the directories and file names it will create create are not the same as your PS scripts?  Why does that matter, since Oracle is managing it all for you.
One caveat that a lot of people miss.  Let's say you declare your FRA to be at '/u01/oracle/FRA'.  Now, say you specifically specify a backup destination format of '/u01/oracle/FRA/mydb/20201008/'.  Doing so is not using FRA functionality, and those files will not be known to be in the FRA, even though they are there and consuming space thought to belong to the FRA.