To move tasks upwards and downwards, you could use the built-in keyboard commands in the Organize menu (as shown above in this thread). You could also, more unexpected and still undocumented as far as I know, use the keys u and d. There are also some other single-key shortcuts in Omnifocus:
To move task to the top or bottom, you may use a very old script that still works (at least in Mac OS 10.14):
Toolbar scripts: Move to Top/Bottom
I have used Keyboard Maestro to connect keyboard shortcuts to the toolbar scripts.
