Using AS OF TIMESTAMP or DBMS_FLASHBACK.ENABLE_AT_TIME across multiple schemas
Yes you can use dbms_flashback.enable_at_time. This changes your current sessions perception of the whole database (all schema). 

But keep in mind that you would have to write changes in a different session, so you cannot use insert from those statements.

There is a current video from Connor as of AskTom fame on this: https://youtu.be/lmrWmH1vMPc

Another option is, if you want to have a consistent but current view you can set your transaction isolation to read only and begin a new transaction to get read consistency across multiple queries.

If you want to mix both read and write then you would need to use ‚as of` for the read subquery. The problem you have with that (changing  many statements) can be easily solved by actually generating those statements automated.