This is a suggestion for a recipe mode similar to catalyst, but instead the catalysts are the same for everything - "Quality upgrade pack (uncommon/rare/epic/legendary)" that raises item of previous level to the specified quality level. In this mode it would probably only make sense to have quality for placeable entities and equipment.

Would this make more sense as a mod setting or as an external add-on?

I think that idea was already used in: https://mods.factorio.com/mod/promethium-quality?from=search
But if you want you can fork my mod and add quality upgrade recipes you want manually - each will need a balancing for cost. You don't want to have to spend same amount of resources for solar panel as for biolab, right?
This mod can be used as a framework for complex and different quality recipes, compared to vanilla game. It does the hard part - emulating quality system and it's also customizable.
Even if you don't have modding experience you can still change a lot of things easily - /default folder in mod contains all files that can be changed.
I haven't worked much on balancing recipes for quality, so you can make a suggestions about them.