NGAD is just a program to test technologies to use in a 6th generation fighter. It is not a program to build an actual 6th generation fighter.
It is basically similar to a 6th generation version of the Shenyang FC-31 but within a much larger program.
The F-22 was cancelled because of politics. The F-35 program was consuming gargantuan amounts of capital and they diverted F-22 funds to it to try to solve the problems. Because the Soviet Union had collapsed the need to have increased numbers of F-22 aircraft was reduced and China was not at the time considered to be a threat. The F-35 was considered to be the fighter which would bring jobs and export sales. We are in 2021 and they still haven't managed to push the F-35 Block 4 software out of the door. Now they want to rewrite the whole software stack for NGAD? Good luck.
While there were plans for increased range F-22 derivatives once China became a visible threat, like the FB-22, or more agile variants of the F-22, like the X-44 MANTA, both of those were scrapped. The FB-22 was scrapped to increase funding of the Next Generation Bomber program.
As for the F-22 being considered hard to maintain, they had a long time to solve whatever problems it had. It entered service in 2005 a decade and a half after the YF-22 first flew in 1990. Consider that next time you see people joking about the Su-57 being "delayed" or some other rant like that. The Su-57 took 10 years from first prototype (T-50-1) to first production, not 15 years like the F-22.
Now people seem to think these new programs will magically solve the problems of the existing stealth fighter. I think that is bullshit. They will just compound on existing problems. The next fighter is not going to use mature or perfected technology, they are going to basically redesign everything from the bottom up, engines, airframe, software, etc. What makes people think development will be trouble free?