How can I improve my cable routing I need to re-cable* and rewrap my bars anyway.  I'd like to take the chance to improve the cable routing for a couple of specific things:

The gear cables catch on the dynamo light mounted on the fork crown in tight turns.  This can cause the barrel adjusters to creep, losing my low gears (I've locked off the rear one as there's another, but can't do that for the front).
I sometimes use a bar bag. At the moment I have to fit the bag behind the cables (brake and gear), then pack it on the bike.  The cables restrict what I can put in there.  If I want to use the bungee net on the outside of the bag, I have to remove it and relace it over the top of the cables.

So I'd like to re-route the cables, but I'm not sure what my options are.
Here are front and side views, without the bar bag:

And a front view with the bag empty and roughly fitted for show

The light mounted on the bars, that clashes with the bag, is only there for testing, and isn't a concern.
If I extend the cables to give more clearance for the bag, they'll be even worse for hitting the dynamo light.  Part of the problem is the cable stops with barrel adjusters on the headtube.
I suspect there are things I can do without more accessories, but there might be some small  device that makes a big difference.  Ideally the cables would route behind the bag, but they're not doing that with those cable stops.  The rear brake cable (left hand as I'm in the UK) also doesn't work too well for this, but would be easier to deal with.

* I snapped a gear cable inner yesterday, and there was so much friction when I fitted the replacement at the roadside I couldn't shift at the back.  For 200km until I got home.  The outer seems to be the issue.
You want to get the housings situated closer to the frame so there's room to put your bag in front.
They curve pretty far into that space now. To get them as tucked as possible:

Don't zip tie them together like that.
If your STI model is one that has two routing options for the shift housings, use the one that puts them behind the bar.
You can route the brake housings (as well as the shift if the above option doesn't exist) so that under the tape they slope downward, and then exit around the 6 o'clock position on the bar, rather than in front like they do now.
Make them all as short as possible. They don't look like they are now. This is usually defined as the minimum length to still have some slack when the bars are turned as far as they're able in either direction.
Corroborate that you can't cross the shift cables under the downtube (usually not possible with this style and location of stops). If you can, doing it that way may make the housings protrude less.

The bag you have isn't made to work with a handlebar bag rack and decaleur, but eliminating this kind of problem is one of the things they do, along with the sorts of handlebar bags they're used with. They could also provide a better place to mount the light as well.
For the light, if the above doesn't get the housing out of the way, one avenue is switch to metal fenders, use a bracket to get the light all the way forward on them, and put an additional strut behind it for stability. A number of old and new rando bikes have this sort of thing. There's even now an off the shelf product that does it. Do not forego the additional stabilization of either a strut or a rack, and don't do it with plastic fenders.