Could you elaborate on how ADHD affects writing e-mails, if you do not mind? I expected writing e-mails would be more comfortable for someone with ADHD because they can take their time with the e-mail. But then again, the time is limited and maybe this freedom to write in your own time means that you write a single e-mail way too long, rewriting it, … I do not know much about it. I would welcome some insights on the topic.
Nevertheless, I like some of these suggestions very much. I might try and see if I can improve some phrases in my e-mails with these as well.
Beside concrete suggestions, I would suggest having a look at the applications you use. (Decide whether you actually use and need them first.) Search for the application on AlternativeTo or similar sites and look for a FLOSS alternative there. And also search the internet for general FLOSS replacements for the use-cases of the applications you use. No need to rush anything. Even replacing one application a week is good progress.