Given that you have already confirmed the Apple Account, Wi-Fi, Bluetooth, Handoff, and proximity—and that Handoff itself works—the remaining troubleshooting can be kept fairly short.
Apple’s current guidance is to install any available software updates and restart both devices.
Since you are on macOS 26.6, check System Settings >> General >> Software Update and install anything offered.
The latest version of macOS is 26.6.2. Learn how to update the software on your Mac and how to allow important background updates.
Apple’s official Universal Clipboard troubleshooting is here: Apple Support — Universal Clipboard.
After updating, restart both the iPhone and Mac and test again using a simple piece of text in Notes.
Copy it on the iPhone and immediately press ⌘V on the Mac. Apple notes that Universal Clipboard content is available only briefly, until something else is copied. (Apple Support)
Something not mentioned earlier
If you use a VPN on either device, temporarily disable it and test again.
Apple specifically notes that a VPN configuration that prevents local networking can interfere with Continuity features. (Apple Support)
You can also briefly turn Handoff off and back on on the iPhone under Settings >> General >> AirPlay & Continuity >> Handoff, then restart the iPhone.
Apple confirms that Handoff is required for Universal Clipboard. (Apple Support)
Since your Mac >> iPhone direction works while iPhone >> Mac does not, and you are already eliminated the normal configuration issues, I would avoid repeatedly changing your Apple Account, Wi-Fi, or other settings unless the above steps change the behaviour.
For reference, Apple’s Mac-specific instructions are also available here: Apple Support — Copy and paste between your Mac and other Apple devices.