Well to keep things simple I would just

1. use incremental backups, a level 0 (complete) backup per week, monday to saturday level 1 differential incremental backups and with block change tracking if possible

2. skip read only tablespaces, something like

backup database skip readonly plus archivelog delete input;

if you need to put the tablespace read write again just make sure dont put it to read only again before you perform the backup for the tablespace and if you have major application changes just perform a full backup (get rid of skip readonly clause from your backup command for a few days)

3.Dont worry about FRA, you can store your backups anywhere you want