You can disable the standard order confirmation email from Admin-
Stores > Configuration > Sales > Sales Email > Order.
Here $this->orderSender->send($order, true); called which extends abstract class method() which check if Order confirmation email is enabled from backend or not so we need create plugin for that
As you specified in your question that you have disabled Order Confirmation email from backend, I have assumed the same and write code in such a way that email will not be sent twice even if you have enabled Order Confirmation email from backend. So, Order Confirmation Email will only send from the success page when you have disabled it from the backend.
My findings was that asynchronous sending was On and while email communications was turned Off the values of field email_sent and send_email should have been set to NULL just as it is on
When email communications is turned off and asynchronous sending is On next time you switch email communications back on be sure to update the fields ...
Dot mailer is the Specialist for email on Magento 2. That might be helpful to contact and see the service they offer.
Either way, it strikes me the issue is more about your dns has now not a good reputation. How about starting from scratch using a different ‘from emails’.
Surely you should be able to hire a freelance Temporarily to try out isolated scenarios ...
Simply create a event observer to do prevent the order confirmation email
<observer name="Test_Module_event_email_stopper" instance="Test\Module\Observer\EmailStopper\OrderEmailStop" />