Hey Tom, you didn't ask for any ideas about your rear shifter cable routing issues, but I can't resist troubleshooting. Feel free to ignore any/all suggestions. You have a couple options that might help, and a few more that will be a cure-all. But you'll have to change-up or give up some stuff for many of them.

First of all, I reckon the front shifting isn't impacted much because the cable/housing is shorter, so there's less overall friction, and you only have to shift across chainrings, so the additional friction isn't as critical. You don't need nearly as much precision in the front as you do in the back.

Mods from easiest to hardest, kinda, maybe?

1. Find a way to mount that bag lower, so the top of the bag is below the shifter cable exit
[might help]
2. Replace the front bag with a different front bag that mounts lower
[might help]
-both of these options will make it more difficult to access the front bag while riding, because the shifter housing will be above the bag top-

3. Change your shifter cable routing to under-bb, then criss-cross the shifter cables under the downtube; front housing goes to left side, might make the bend easier/better?
[might help]
-if you don't want to drill/tap your bb shell for an under-bb cable guide, hard to do this well-

4. Replace brifters with
a. later models that route shifter cable/housing under bar tape; run cable/housing all the way to stem
[will cure]
-maybe most expensive option-
-fairly extensive labor-
b. bar-cons, run the cables/housing under the bar tape all the way to the stem
[will cure]
-requires new shifters, and new brake levers if you don't want the now-useless shifter bits hanging uselessly-
-relatively extensive labor-
-step backwards for convenience; maybe you like brifters better-
-plus: could be cheaper than new brifters, especially if you already have spare brake lever on hand-
c. downtube shifters!
[will cure]
-even bigger step back for convenience-
-plus: completely isolates shifting system from anything you do with bars/stem/bag/etc

5. Lose the front bag and use a saddle/seatpost-mount bag instead
[will cure]
-requires new bag, and you can't access stuff while riding-

6. Install a lightweight rear rack, and use a rear rack bag
[will cure]
-requires new bag and new rack-
-more installation work than a saddlebag-
-heavier overall-
-plus: less impact on handling than a saddlebag, in general, because weight sits lower-

I thought about using some kind of brake cable angle guide ("noodle") like are used on v-brakes, but I can't envision a way to make that work.

Did I miss anything? I won't force Tom down the single-speed rabbit hole (Not a problem if you don't need to shift!).