Overall, I think you did a good job. Overheated the tube like Doug said, but you got good penetration.

I suppose it's better to put too much filler in that not enough. And since you slotted so far in, it might actually be necessary. I think I would make the slot no more than 1/2" or so and cut down the rest of the dropout tab so it fits inside the stay. Structurally, it's better to have a slot that isn't so long. I think those bubbles are flux. They aren't going to hurt you. Since the outside of the tube is pretty crusty, the flux on the inside had to work pretty hard to keep the inside of the tube clean enough for the filler to wet out. I don't see pinholes on the outside, which indicates to me that your filler wasn't horribly dirty. But I do think the bubbles are partially because you overheated the tube. I'm a little surprised the filler didn't push them along to the far end of the dropout.

Once you get the dropout up to temperature, the tube will get up to temperature very quickly. I'm curious if you heated them both at the same time. Best to avoid that.

Yeah this is probably accurate haha. I was actively trying to heat the dropout first but maybe to your previous point about the tab being too long, it was hard to get the end of the dropout tab red without heating the tube too much. I'll try again with a shorter tab and take Doug's suggestion to scotchbrite the filler rod.

If you didn't clean it at all, I'm impressed you didn't have more problems with pinholes. Years ago, I had some filler I bought at the LWS that was really dirty, and I never thought to clean it. It really didn't work well, I had problems getting it to wet out. I started buying gasflux c-4 and it was so much better. Recently I went back to using the LWS filler and once you clean it, it works just as well as the C-4.
I use shop cloth sandpaper to clean my filler, that is good to have available. I have 1" and 1/2" rolls, the 1" seems to be more generally useful. It's emory cloth, but often sold as "shop roll"

I only cleaned the filler rod via acetone wipe, I did not sand or scotchbrite it. I did remove the mill scale from the dropout and both inside and outside the tube with 320 grit, and followed with acetone wipe.

I only sand, I generally avoid chemicals. There is mill scale on lfb filler that isn't going to come off with acetone. Silver is usually clean enough, I think. I have never cleaned it. Maybe I should.