As we can already tweak explosive biters' explosive res, I'd like to be able to tweak down their fire res to like 98% so they eventually die from the flamethrowers when they target a wall from max range, and the only thing that can reach them is a flamethrower. Maybe a setting for each res ? or even quicker, link all res in one setting.

You can change this manually by opening the mod zip and looking for values.lua, then change each resistance you want to modify to whatever value you prefer, or copy paste the "explosion_resist" variable so they are all controlled by that one setting ingame.