Ummmm, I'm puzzled.
I too was disappointed about the lack of bleed, having come back to learn Designer after some time away. Reading here after this I think that the solution is actually there, but is simply not visible...
I set up an A4 page, and then turned off "Clip-to-Canvas" using the /-key so that I could see objects off the page.
I then added 5mm bleed guides using the 'Guides' panel that are outside the page (-5mm, 215mm etc), and they are not visible, but a placed object snaps to them just as to any visible guide, and the vertical/horizontal (Red/Green) snap-guide indicators appear to aid alignment as well, so bleed images are a cinch too.
The only missing thing is a visible bleed guide, but any suitably coloured page+bleed rectangle gives easy reference to work with and the functionality is all there.
I understand that negative margins aren't possible, but negative guides are, although they aren't visible.
I've attached a sample file.
Bleeding.afdesign
Muppet64 got a reaction from jamesholden in Bleed lines not showing on document Affinity Designer