After a very long search, I settled on Thunderbird Mail with just leaving Chrome open with Google Calendar.
"There be dragons" when it comes to line breaks in plain text email. Apparently there's a header field "format = flowed" and there's a requirement for lines in text email not to be longer than a certain amouont. Ideally, the receiving email client will reflow the text and soft-wrap it as expected, but for some reason emails sent from Thunderbird don't do this (on K-9 Mail at least). Mails sent from Apple Mail do.
For now I am using a Thunderbird extension called Outgoing Message Format to send emails in HTML. More testing is warranted.
Update: Plain text sent from TB:
- Does reflow correctly in TB
- Does reflow correctly in Apple Mail
- Does not reflow correctly in Android K-9 Mail
- Does reflow correctly in the GMail web interface
I think I will not worry about it and just let Firefox handle the format as Auto, since the majority of people will probably see the message reflowed.
It's still strange how when sending plain text from Apple Mail, K-9 reflows it correctly (this even happened when I forwarded a message originally sent from FF through AM). Maybe K-9 is looking at a different field in the header to see if a message is sent by Apple Mail? I can see the format=flowed in messages sent by either client.