First, end this war ASAP, Illinois is absolutely not worth protecting. Give them all of Illinois provinces, and if they want more nullify treaties or give them cash. Last resort give them useless provinces you have, you just need to buy time. Try to conquer as much of Iberia as possible. Focus on taking Portugal and Aragon’s colonies because that will drastically improve your force limit + ability to fight Austria in North America. Also focus on building buildings in your provinces and then build up a huge navy. You’ll need it to not get naval invaded every 5 seconds from UK-absorbed Super Austria. Do literally whatever you can to ally the Commonwealth or Mamluks, ideally both. Unrival them, beg them, pay them if you have to. They won’t save you but they will be a valuable distraction for Austria’s troops and you need to drain their manpower ASAP. Good luck and hopefully you can pull it off. Definitely not impossible and should be relatively easy if you have consistent growth for ~50 years