Portables and duct work really do not go well together. The length of hose you get with the units are the total length they recommend. You may shorten the life of the unit? I would try to exhaust them another way. IMO Good Luck
Yes sadly the blower fans are not that strong and any restriction will cause back flow pressure for sure. I went through five units this way in a 2 year term. Are you closer to your chimney? Hot air rises naturally and if you use it, it would benefit you.
Had one last one season or so due to the long exhaust, I even boosted it with a fan at the end of the run, dunno if that helped or hurt but either way that ac was toast.
I agree is says in the "book" not to over stretch the duct work adding more will likely cause you problems. can you duct the cold air coming out of the a/c that way?
It's not only the size of the ducting - its the amount of bending that causes problems. Yes you can use larger ducting (also add a high cfm fan to push air into this duct). Try to run straight ducting. Each bend - even a small one - will significantly reduce the amount of airflow in the tube. If you have the money then get a ductless minisplit.
Got a ductless in my flower room, this is for my veg, and i can't afford another mini split right now. Basically I was planning on coming out the portable with a 90 through the floor then another 90, then all straight after that.