1) For what you are trying to do, have you tried scheduling an ARCHIVE on
the weekends instead of a backup; you can specify a management class with a
retention period of 4 weeks and get the same results without playing the
now-you-include-it, now-you-exclude-it game (which you will usually LOSE
unless you declare a second node name for the client).

I do that for one of my servers, for .bkf files, like this:

"Action" = ARCHIVE

"Optons" =
-archmc=ARCH_6WKS -desc='Weekly Archive'
"Objects" =
"Objects" =
 d:\directorypath\*.bkf

2) I also determined that a HUGE chunk of my backups were due to .pst files.
Users tend to grow them larger and larger without end (we've had several
people approach the MS 2 GB limit!).  And, as you said, they appear to
change the WHOLE file every day.  We solved that VERY nicely by turning on
SUBFILE backup.  Works great, no problems.

We turned it on for ALL our clients files.  (Our .pst files are on people's
own machines, not on the server.)  BUT if you want, you can specify an
INCLUDE statement:  include.subfile ?:/.../*.pst

So that only the .pst files will be subject to subfile processing.

I have been running subfile backups on ALL our client files since November.
Have had NO problems with it.
(Have used it with the 4.2.0 Windows client, never tested a lower version.)
You can turn it on from the server in a client option set, if you choose.
Dropped my daily backup load by 25-30%!

I don't recommend using SUBFILE backups on ALL files unless you are using
collocation; because each RESTORE will restore 2 files instead of 1 (the
base + delta).   But we use collocation anyway, so it's painless.

This could be a VERY painless way to solve your problem!