I attempted to try this mod with all BZ mods, Brass Tacks, If I Had a Nickel, Space Exploration, Krastorio2, and some other mods.
I get a message saying:
Failed to load mods: Error while loading recipe prototype "zinc-plate" (recipe): Key "icon" not found in property tree at ROOT.recipe.zinc-plate
Modifications: Brass Tacks > MetalsForYou > Space Exploration

It appears to load properly without MetalsForYou.

Should be fixed now

Hmm. It still seems broken as of 0.1.2. I've also tested it with only Space Exploration (plus dependencies), Brass Tacks, and MetalsForYou, and the error still happens, so I don't think it was an issue with my original complicated modlist.

I think it is fixed now

So it seems to be still not fixed. But now the error is different depending on what mods I load it with. When I load with only Space Exploration, Brass Tacks, and MetalsForYou it gives:
Failed to load mods: Error in assignID: item-subgroup with name "engine-components" does not exist.
Source: turbine-blades (item)

When I load with SE, Krastorio 2, all BZ mods, Brass Tacks, If I Had A Nickel, and MetalsForYou it gives the error:
Failed to load mods: Error while loading recipe prototype "tantalum-capacitor" (recipe): Difficulty normal: Key "name" not found in property tree at ROOT.recipe.tantalum-capacitor.ingredients[1]
Modifications: MetalsForYou > Space Exploration

and when I try to load with the entire modpack I'm making, which also includes Them Thar Hills and Lasing Around, it gives the zinc-plate error I reported at the beginning.

By the way, thanks for your speedy updates; I'm sure it can't be easy to check against all the mod combinations that might or might not be present.

The engine-components subgroup is only added by If I Had A Nickel.

The zinc-plate error can be fixed by either commenting this line in recipe-modify.lua:

util.add_product("zinc-plate", {name="indium-plate", amount=1, probability=0.6})

or by enabling Bismuth, which affects the same recipe. This suggests your add_product function is not setting the main_product of the recipe - recipes with multiple outputs need either an explicitly defined icon or a main_product to grab the icon from. (Oddly, the indium byproduct does not actually appear in the game when the line isn't commented and bismuth + K2 are loaded.)

The tantalum capacitor error is caused by this line in tantalum-recipe.lua:

tantalum_capacitor_ingredients = {{mods["bzlead"] and "lead-plate", 1}, {mods["manganese"] and "manganese-plate", 1}, {"tantalum-plate", 1}}

If bzlead or manganese is not installed, the first part of the ingredient specification is omitted, and you have an amount without an item. You want the syntax to be

mods["bzlead"] and {"lead-plate", 1}

which will omit the entire ingredient if the mod is not installed.

There are also a few places throughout your code where you check for mods["modname"] and "itemname" - this doesn't do what you want it to. "itemname" is just a string. You need to check whether the item prototype exists at data.raw.item["itemname"]

This clarifies some of the errors I was getting, because I'm not using Manganese or Bismuth.

I can now confirm it loads when I add both Bismuth and Manganese. Now that I can load it, I'm checking out some of the intermediate products and they look quite interesting. However it appears that "thoriated filament" has no uses according to FNEI.

The engine-components subgroup is only added by If I Had A Nickel.
-That explains it, I was sure it was from AAI industry

The zinc-plate error can be fixed by either commenting this line in recipe-modify.lua:

util.add_product("zinc-plate", {name="indium-plate", amount=1, probability=0.6})

-I will see if I can fix it

The tantalum capacitor error is caused by this line in tantalum-recipe.lua:

tantalum_capacitor_ingredients = {{mods["bzlead"] and "lead-plate", 1}, {mods["manganese"] and "manganese-plate", 1}, {"tantalum-plate", 1}}

-Yupp I missed that

There are also a few places throughout your code where you check for mods["modname"] and "itemname" - this doesn't do what you want it to. "itemname" is just a string. You need to check whether the item prototype exists at data.raw.item["itemname"]
-I see, thanks for the help. You are very good at explaining things!

(not sure how this forum format works)