Custom Email Actions and Send through External Email Services / Send through Office 365

I've got "Send through Office 365" set up such that emails from Salesforce are sent through Office 365 as per https://help.salesforce.com/articleView?id=email_enable_send_through_external.htm&type=5

The issue I am finding is that this only works with standard email actions and not custom email actions  (i.e actions where Action Type = Send an Email). If I use a standard email action the email is sent from Office 365 but when using a custom email action it is sent through Salesforce even though I have specified "Send Through Office 365" in My Email Settings.

I've tried searching online but can't find any results for this and nothing in the considerations help article which suggests I'm missing something obvious or this is not possible. If the latter I am surprised this has not come up before. Does anyone know more? 

The use case is we need to create loads of custom actions to do things like set the subject, use a specific template and set predefined values for fields. 

Thanks!