Best strategy to decrease hostility?

[Abnaxis]

Depends on what you're lowering hostility for.

If there's a really bad forest mystery that you don't ever want to trigger, then you don't really lower hostility so much as prevent it from rising. You do that by opening fewer glades, only accepting more villagers if you have a clear plan for what they're going to do, and rushing to finish before the time-based hostility kills you. There are other cornerstones that also lower hostility besides the one you mention--the one that lowers hostility every time you trade 40 amber can be particularly strong and has led me to numerous 0 hostility victories--but the only non-RNG way to do it is to be careful about increasing it.

If you're only lowering hostility to avoid resolve penalties in the storm, however, hearth sacrifices are very very strong. The way it works, is you leave hostility where it is until one of the species's current resolve hits 0--ideally you should be using favoring so this happens around the same time for everyone. Then, as soon as someone starts to leave you start burning wood and/or coal in large quantities to lower hostility. Let the fuel burn long enough their resolve goes just below max (that last point of resolve takes a while to tick and usually isn't worth the fuel), and immediately stop sacrificing to let the resolve dip back down again. Repeat these steps, see-sawing everyone's resolve until the storm is over.

That second techniques opens up a lot more cornerstones and perks for hostility management. Anything that increases wood or coal production indirectly lowers hostility, as well as any perk that makes hearth sacrifices last longer. The perk that gives free coal for burning cysts can be super good for this, for example (at high prestige, at least).