We'll have to agree to disagree on the brass (bronze) filler. Bronze may not be as strong as the steel, but you can use more of it to compensate. That's why a bronze fillet brazed frame is as just as strong as a lugged one.

Crack propagation is a study in fracture mechanics, a field that I haven't studied much, but what you're trying to do is remove stress risers. In many cases you actually want a softer material. I wouldn't fill this hole with brass, I'd make/machine a small plug and silver braze it in. That keeps the heat affected zone down to a minimum. Filling that large a hole with brass filler would take some skill.

I did something similar to this a few years ago on a vintage Falcon frame.




Defluxed, filed and sanded the outside. Luckily a new wrap around Falcon decal hid the repair and no repaint was needed.

But, again, odds are nothing will happen on the OP's frame if he leaves it alone and just rides it, however the odds of a crack propagating are much, much higher with the hole (my 2 examples I mentioned earlier, for example). Ride it or not - that's a personal decision. I'm not saying it's a bad choice to not fill it. Saying it won't crack is just guessing, however.