Hi Gus,
Could you provide a screenshot of your custom activity form?
Are there separate lookup fields for contact or account?
Because without them, when you create an activity from a contact, the Regarding field of the event is the contact, so there is no lookup field for the related account.
In general, we can achieve your needs through field mapping or workflow. Using them does not require you to manually select an account, the account field will be automatically populated.
If I know your custom activity form, I will write a demo for you.
If there are no other account or contact fields other than the Regarding field, then you could only use the quick view view to display the account.