The original version of the sentence was correct:
I have Sodium and Potassium.
This type of element (i.e. elements considered to be metals)...
Sodium and potassium are a single "type of element" (considered to be metal), despite being more than one element, so "element" should be singular.
Even if your list was many longer, it would still be a single "type of element":
Sodium, lithium, potassium, rubidium, cesium and francium are the "alkali metals".
This type of element is defined by ...
If, instead you had:
I have sodium and potassium, which are metals, and helium and neon, which are noble gasses.
This is when you get to pluralize, and you would make both parts plural.
These types of elements...
The two "types" here are "metals" and "noble gasses".
The same is true with your other examples:
He prepared some french fries and insomnia cookies, assuming that this type of (crispy) snacks would fit my appetite.
In this example, there's still only one type of snack... crispy, so this should be written:
He prepared some french fries and insomnia cookies, assuming that this type of snack would fit my appetite.
And:
I am recently addicted to Triumph of Death and Tessellation; when I listen to this type of (metal) songs, I feel alive.
There's only one type of song... metal.
I am recently addicted to Triumph of Death and Tessellation; when I listen to this type of song, I feel alive.
Now, to add to this, it's not uncommon in actual use to find examples with mixed singular and plural forms like your example. Right now, I'd say they're ungrammatical but whether they remain ungrammatical could be considered to be in flux.
For the time being, if you make sure to ask yourself, "How many types are there?", you should be able to be sure to use the correct form.
This earlier question addresses your issue as well:
And here's another one that might help:
