How to get my Note 9 to read aloud incoming email ONLY from one specific Gmail account? Any way to do this? Tried various apps but they read out all email accounts. Want to setup my phone to read incoming emails out loud from one specific user too if possible.
FairEmail
since version 1.1254 released on 17 Jul 20, allows you to read out aloud  mails from selected sender(s).
A big shout out to Marcel, the developer who accepted this feature request and implemented it. I have tested it thoroughly over the last few days (I had a test version much prior to the formal release) and it works faultlessly for me.
Here is how you do that :
Create a rule:
Go to the inbox of your primary account, long press → Edit rules and tap on the + icon to create a new rule.

Set order to 1 (lower the order, higher the processing priority). Make sure the rule is enabled.

In the Sender Contains  enter email address of the sender.

In the Recipient Contains enter email address of the recipient.

Type and not copy-paste email addresses (at times copy-paste has invisible blanks which can break the expected behavior)

Leave Subject Contains blank.

Select Action as Text to Speech

Press Check setup. If it says OK, you are set up, else install voice data(TTS engine).


automation approach had mixed results. You can read about them in previous version of the answer
Notes

You need the paid version of FairEmail.

The rule should be created in inbox folder of the recipient account.

At the cost of repetition, leave the subject on the rule blank

This is for one sender. If you want this action for more senders, you need to create more rules as explained. You need to create one rule per sender (tip: you can long press a rule to copy and edit, instead of starting all over again) . 

The mail text that is read out is limited to the text that is contained in the notification text, that is 500 characters (Android limitation of notification text size). IMO, this is more than enough to get the message in full most of the times (unless it's a very long mail).