OK, what I used forts for:
- blocking rebels (as I cannot be everywhere with my army)
- blocking in coalition wars
Army true, again, overspent on army due to rebels and multiple front wars...
One more push in India and will do the cost cutting...
Why would you care about rebels? So what if they get to be unblocked for a few extra months? They will occupy 2 extra provinces? That's not worth stationing a fort there forever.
And there is a progress counter for rebels, you can see when they are at 70 - 90% and not start a big war far away.
As far as "slowing down coalitions" goes, you shouldn't get attacked by a coalition in the first place. Two thirds of a successful war are won before you declare it, using diplomacy. As the Ottomans every war should be you declaring it on a weaker enemy.
I also see most of the advice is focused on "build manufactories and conquer India". Manufactories are great long term, but they won't help someone already in loans. And conquering will only help on the margins if you get new lands along with new forts.