I’ve watched the various forums for a reasonable answer to this request and seen the reasons why people say it is difficult to impossible for this to be implemented at present.
It seems to me that the solution is for Affinity to produce a programme that is purely for ePub output.
Ideally it would be possible to open an affinity publisher programme in it and manipulate the design.
For example; on import the programme would, ask what text you want to import, know how to deal with headers and footers. Strip out extraneous commands. Prepare indexes. Have protocols for displaying graphics, photos etc.
All in a wysiwyg way and flexible enough to take into account the various kindles, tablets and mobiles that the end users have.
I would be prepared to pay for this.