- Always seek staying at +3 stab.
- Respect manpower, attrition and war exhaustion. Going 18k siege on enemy land is too much, especially if you do not have owned/controlled lands bordering that siege.
- I assume the 42k you talk about is your manpower? name it manpower, it confused me in the beginning.
- Also, creating armies during war shouldn't be something to depend on, production areas could get attacked, and production itself is time-consuming thanks to the usual war exhaustion, also it will leave less manpower to reinforce injuries & produced units will need to regain morale & group up first.
- Don't start a siege unless you're confident it will most likely hold. Whether it's because you killed all hostile armies, or pacified them at least, or have a guard force near that siege, I don't know and it's not my business, just as long as you're sure that a months/years long siege will not be interrupted in a way that could have easily been avoided or a way that was obvious.
- +1 on going for coastal provinces, I recall base tax being the involved factor, in addition to navy buildings. Also, crippling their economy in general will limit them too, cause it's what factors in for colonial intentions (e.g. having infinite manpower wouldn't really help colonization work, thus it doesn't really factor in).
- +1 on Portuguese cored region, & the similar.
EDIT: Losing 18k, at 1440, regardless of nation, is painful, I would never go far as labeling them "expendable".