I’ve been in the same boat as you with the Forge of Foes kickstarter but I ultimately ended up backing it. My main reason for backing it is I still run 5e for my group even though that will likely switch over to ICRPG once we end the campaign. For ICRPG, the chapters dedicated to mechanics and tables to generate won’t be super relevant.
That being said, I have all the other Sly Flourish books and I reach for them quite a bit when I am preparing just because they provide a lot of insight that is system agnostic. I really like the way the books get to the “why” of designing things a certain way and don’t just dump a bunch of stat blocks and say go crazy. I get a lot of similar vibes from Hankerin’s writing which is probably why I like the two so much.
One thing I would recommend is to download the Quickstart PDF (It’s the button that says 30-Page Sample PDF) from the campaign page and give it a read. Half of it is dedicated to building monsters for 5e but the other half is about how to place your monsters in your world and that’s the stuff you might find that makes it worth backing.
Hope this helps!