Walkthroughs of standard maps, focusing on SPEED.

It was my mistake in the past that I underestimated diplomacy so much.

Diplomacy is especially useful on random maps and on custom maps where the neutral settings are not defined – then, indeed, about half of the neutrals can join. But on other custom maps, the neutral settings are almost always rigidly set as hostile, with the exception of those who must join as part of the storyline. Therefore, I myself, almost never playing on random maps, neglect diplomacy. It has only one other useful property: it lowers the hero level required to visit the Library of Enlightenment.
The complete algorithm for calculating neutral recruitment is in the latest version of PhysMig, section 5.6, Diplomacy and its application.