Then

OPEN SYMMETRIC KEY MyKey
DECRYPTION BY CERTIFICATE MyCert;
GO
SELECT
PlainTextData
,TextData_encrypted AS EncryptedTextData
,CONVERT(VARCHAR(15), DECRYPTBYKEY(TextData_encrypted)) AS...

What was the data type of the data you encrypted?

If the correct certificates are installed into master, then the restore is identical to a non-encrypted backup afaik.

If the staging tables are a decent size (over a million rows), consider creating them with a clustered columnstore index.

There's no 'for each table' construct

You can write some complicated dynamic SQL to generate the ALTER  TABLE  statements automatically, check them all to make sure they're correct and...

The second SQL has to go and create the files, which takes time.

From the numbers, I'm going to guess you have external memory pressure. Something outside of SQL is consuming memory, and so SQL cannot increase its memory usage. That would be...

RPC is remote procedure call. It's the call from the application.
So, if app calls proc 1, which calls proc 2, proc 3 and proc 4, you would have got...

There's no way to 'thread' within a stored procedure. You can have multiple procedures that each work on a subset of the data, run them from jobs or different connections.