Are you driving or flying? Makes a difference. If flying seems like the North Cascades are a good choice since the extra time to get to Seattle, vs SLC is not that much more. If driving, there is another very long day or two days each way. Others will have to chime in on actual hikes in the Cascades since I have not hiked there in over 40 years. Sept. is after school starts so I doubt you would have much trouble with permits, except for the really heavily used areas.

Also, are you willing and able to do off-trail travel? I am not talking major bushwhacking - but fairly easy but off trail (need to be really good at navigation). If so, many options open up. From the no permit standpoint, cannot beat the Wind Rivers. There are lots of areas there different from your previous trip. And there are a lot of non-JMT loop trips in the Sierra, particularly if you are willing to do some off-trail sections. Again, in the Sierra, permits are easier to get in September (as long as you are talking post-Labor Day).

Also, how many do you mean by "some buddies"? Four, two, ten?