Having recently become frustrated at cut & paste of characters for Old English, both original characters and macron-adorned vowels I settled on using AutoHotKey v2 with Windows 11.
This requires a configuration script, which is inside this zip file: Old English AHK file
Simply double-click on that script file after installing AutoHotKey and the following key combinations (“#” denotes the “windows key” and capitals are obtained using the Shift key as usual) just work:
To add a macron, hold Alt then the tap the chosen vowel -> ā, ē, ō, ī, ū , and ȳ
#+a -> æ
#+Alt+a -> ǣ
#+A -> Æ
Alt+t -> þ
Alt+T -> Þ
Alt+d -> ð
Alt+D -> Ð
Alt+w -> ƿ
Alt+W -> Ƿ
The Alt key can also be used to get these:
Alt+c -> ċ
Alt+g -> ġ
The appearance of the characters will still depend on your choice of font, of course.