I love this idea! Some thoughts:

1. The evolution rate (0.0-100.0) should increase linearly based on the amount of scrap mined.
2. The quality level drop chance should increase exponentially. You can use round(maxquality * (10 - SQRT(100 - evolution)) / 10), which will add the chance of higher-quality scrap when the evolution rate hits 19, 51, 75, 91, and 99 — if the vanilla value of maxquality is used. (maxquality is needed because there are mods that increase the number of quality levels.)
3. I would make defensive bot quality increase at the halfway point to the next scrap quality increase. (e.g. with the vanilla maxquality of 5, bot quality would increase from 2 to 3 at an evolution rate of 63, halfway to scrap quality of 3 at 75.)
4. The lifespan of defensive bots should increase (exponentially?) with quality, so that, at bot quality 3(?) or 4(?), the next wave of bots will spawn before the last one has expired, forcing players to get more serious about defenses if they want to be able to mine that better-quality scrap.

Does that sound reasonable and doable?

I was going to do something close to this, probably by artificially increasing the quality +% on any nearby drills.
One limitation here is that I assume that 100% quality bonus, we get 90% uncommon, 9% rare, and so on, we can't get more than that.
The alternative would be to actually replace some of the scrap resource under the vault itself, I haven't tested that yet.

One thing I'm unsure about is what to do with quality levels that are not researched.
I guess the drills would just not output above it, but I have to make sure the defeated bots dropping recycleable items don't bypass the limit.

One thing I'm unsure about is what to do with quality levels that are not researched.
I guess the drills would just not output above it, but I have to make sure the defeated bots dropping recycleable items don't bypass the limit.

The easiest thing would probably be to just replace them with the highest researched quality. Either that or don't have the bots drop anything. (They would have taken extensive battle damage after all.)