Is this definitely not a bug in Ashan or SoD_SP? I'm more than sure that I've encountered similar situations where dragons still attack if it's safe for them, i.e., the damage from their breath attack is less than their health.
Okay, I disabled SoD_SP, uncheck the HD+ box, and, to save time, reduced the number of enemy troops by 100 times, and then started the battle with the rusty ones. Here's what happened.
https://www.youtube.com/watch?v=QwTCKE7-1uc
I think it would be the same even without the HD mod. If you want to check, you can do it yourself.
In any case, it's a great idea. It's a pity that in terms of speedrunning, killing Le Strada doesn't give anything (and even the opposite - you can't escape or redeem with Fathom later).
I don't know about speedrunning, I'm not into it, but in a regular game, this trick gives some benefit, because immediately after getting the City Portal, you can learn such useful spells as: Blindness, Hypnosis, or Quicksand.
But again, this battle is more of an experiment, and I don't think that in a normal playthrough, many people will want to have a battle that lasts 15-20 thousand rounds, which doesn't offer much benefit.
But saving the Blade of Armageddon in the first week is a completely different story. Here, the game becomes noticeably easier in some places.