Hi, love your mod. More options are the best kinds of mods, all of this really should just be vanilla.
Is it possible to have a config option to directly modify the bonuses quality gives?
My goal is to only have 3 higher quality levels but have each give a 50% bonus, instead of the vanilla 4 that give 30% (60% on q5). I just think 4 is kinda bloat, plus the numbers would be neater (no double step final tier).
I can disable a quality tier with your mod by reducing the upgrade chance to 0, but I haven't found any mod that lets you really tweak the bonuses.

the quality level itself is the bonus %

So if you want to remove the "Legendary" quality you would just adjust "Epic" to be quality level 5 and possibly adjust "Rare" to be level 3
This would shift epic down 1, and legendary down 1.

Gaining 50% each tier would actually reduce the overall power of quality by quite a large amount if you are removing a tier as well.
Possibly increasing by 100% each tier and starting at 1.25 for uncommon?

That would give you this spread:
Normal = 0
Uncommon = 1.25
Rare = 2.5
Epic = 5

The current vanilla spread is:
Normal = 1
Uncommon = 1.3 (+0.3)
Rare = 1.6 (+0.3)
Epic = 1.9 (+0.3)
Legendary = 2.5 (+0.6)
My ideal would be:
Normal = 1
Uncommon = 1.5 (+0.5)
Rare = 2 (+0.5)
Epic = 2.5 (+0.5)
This would be a buff to quality overall as you can get maximum in one less tier and each tier is a bigger jump. And the numbers would be neater, always a big plus.
But is this even possible to mod in or is it hardcoded?

Where are you getting that information, that is simply incorrect.

Normal starts at 0:
https://lua-api.factorio.com/latest/prototypes/QualityPrototype.html#level

the default settings for this mod provide you with the standard quality levels.
they are 0, 1, 2, 3, 5

https://lua-api.factorio.com/latest/prototypes/QualityPrototype.html#default_multiplier

I'm not talking about the quality tiers, but the bonuses each tier provides.
In vanilla this bonus is +30% so 0, 1, 2, 3, 5 translates to 1, 1.3, 1.6, 1.9, 2.5
I want the bonus to be +50% so 0, 1, 2, 3 would tranlsate to 1, 1.5, 2, 2.5

once the newest patches make it to live, I will be implimenting adjustment points for the default multiplier. This would allow you to do exactly what your talking about. Currently the default multiplier is 1+.3*level.

Just wanted to update you on the status of this, these adjustments are now possible with the latest version!

I saw, thanks! It's perfect.