This might work, I started playing with folder Permissions, which you can access from the three dots on the right hand side of Outbox folder.
This might not work if you have policies set by your employer or IT team, however if you show them the problem they can help you to make it work.
You don't click on the message, just the three dots for now.
The default permissions I have were None to everything in the Permissions for the Outbox Folder dialogue box.
I took a screen shot to remember what the Default settings were.
I changed to enable Folder Owner under the Delete Access group i.e. click the check box for Folder Owner.
Then I exited the Permissions dialogue box and deleted the stuck message successfully.
I think it might be best to reset the Permissions for Outbox back to None or whatever your Default was to start with because I don't know what other impacts this will have.
Best of luck.