I filed a similar report yesterday...
Xcode 26.4 (17E192) Simulators Keep Showing As Unavailable After Successfully Intalling Them
FB22346008
... with sysdiagnose tarball and screen shots. Am also running Tahoe 26.4.
I spent the better part of this evening removing the following items:
/Applications/Xcode.app
/Library/Developer
~/Library/Developer
~/Caches/com.apple.dt.*
~/Preferences/com.apple.dt.*
/private/var/tmp/com.apple.CoreSimulator.SimDevice.*
rebooting my machine, and then reinstalling Xcode 26.4 (Apple Silicon) and the command line tools. I then used xcrun simctl to flush all of my devices and runtimes. I spent a lot of time fighting Xcode because after removing old simulators, Xcode automatically downloads them all again (SMH!). Once Xcode seemed to stabilize its downloads (using xcrun simctl list to monitor everything), I then added the iOS, watchOS, and tvOS simulators for 26.4. So far, things "seem" to be behaving. We'll see whether or not I wake up tomorrow to joy or despair.
I have a 2021 MacBook Pro (M1 Max) and have developed solidly on it ever since I got it. This is the first version of Xcode that has trainwrecked my world since the horrible days of Xcode 6. Ugh!