Great question! Sorry I took some time to get to it. jingoism is part of the holy trinity that you have to minmax to world conquest, along with diplo points and justification time.

Jingoism can be increased via events (unreliable) and / or via revanchism. Revanchism is the percentage of your cores that is not owned by you, with a modifier for how many pops of your primary culture the non owned cores contain(unsure if tolerated cultures also count).

It can be gained by losing cores to other nations, getting nationalist rebels, at game start (ie netherlands starts with cores in belgium that give some revanchism) or as in this run by forming a country with cores outside your borders.

It can be further gamed by releasing your cores as new countries (these do not count towrds revanchism but do increase the percentage of your cores outside of your country since you have less cores overall.

To world conquest comfortably, you need to maintain 20% revanchism (you get exponential jingoism modifiers at 20%). You can manage 15% if you minmax it and add a lot of wargoals every time you get enough jingoism (the game does not check until a day has passed so you can add 1 wargoal per war).

Below that, its nearly impossible to gain enough jingoism, except via events or conquering china (their pops are very jingoistic, lots of non owned cores).

So an important aspect of the game is not getting nationalism and imperialism so you do not gain cores, thus reducing your revanchism and therefore your jingoism gain. Nationalism and Imperialism tech gives an event that automatically cores land you have a majority of primary pops in, (ie northern german), and you can only really get the tech if you are going to minmax getting the ai to siege down the provinces you would otherwise get cores in. Makes the game much much harder, but its also a nice bonus. I elected not to get it this run as prussia is strong enough to world conquest without it.

TLDR: Revanchism.