First up - figure out your budget. Frame repairs are not cheap and skill costs money. It is conceivable that this repair costs more than a replacement frame that does suit you.
It may be that externally-routed cables are all that you can afford - a bunch of zip ties may be ugly, but functionally equivalent.
Second - you need to know if your aluminium can be welded at all. I had a bike which cracked, and the frame was made of 7005 Alloy, which is heat-treated and any subsequent welding would ruin the heat temper requiring a repeat heat-treatment. That bike ended up parted out and scrapped because of the cost.
Find the alloy number from specifications, and provide that to the framebuilder when getting a quote.
Notice the second photo shows a hole with a lip around it - additional material to act as reinforcement.
By comparison the DI2 hole doesn't have that. I'd immediately suspect the DI2 hole had an extra fitment that lived in the hole to provide strength, compared to the longer slot of the cable-hole.
The thing that matters is the thickness of the material at that point, and how far away from the hole that extra thickness extends.
- If the chainstay is already thick, then enlarging the hole to allow a nice flow of cable outer should be fine.
- If the material is thin, or thins out quickly then carving a larger hole will leave the edges unsupported and likely to fail soon.
Exactly what depth of material classes as "thick enough" should be done by a qualified frame builder with the frame in their possession.
A skilled frame builder might choose to replace the entire chainstay, or cut a section out completely and replace it with pre-made heavier walled tube. Another possibility is cut an access hatch, then make an insert to drop into the chainstay and weld or epoxy it in place, close the lid with weld, and then drill an access hole through the repair.
Whatever the repair solution agreed on, the framebuilder has to carry the warranty so their work has to be good.
If you go to someone who might be a good welder, but is not experienced with bike frames, then you might get a sub-standard weld and a frame that fails.
Ultimately, its all about the cost vs the benefit. Sometimes, its just not financially viable.