I think it was Clive who mentioned pouring into another container with a hole in the bottom. It crossed my mind but I had decided on the water bath to decrease viscosity and was concerned that water may get in and ruin the mix.
Sylavski had also posted lots on epoxy as well as Neal plus many others.
Thing is I'm not ignoring what others have written and it all goes in, and you have a plan, but you need to do it for real yourself to appreciate all the subtle aspects.
As for the tropical hardener I was using the 206 not the 205 hardener as this takes longer to set. I actually emailed West systems about what I wanted to do and was advised to use the 206. Thing is that a skin forms after 30 mins or so anyway and any bubbles will then be trapped so I don't think the long set time helps on the bubbles side. Good for self levelling though.
West said laying in one session was best rather than layers but I got the impression from the overall reply that they had not used it for 5mm beds for CNC rails. It is usually for boat repairs etc.

Whilst I wait for supplies to arrive plus some other parts I am rethinking my approach. Looked into vacuum pumps to degas but this is £200 for a small kit. I think next time I will :

Mix gently
Then place in water bath to help bubbles escape
Pop any remaining bubbles
Slowly poor but start in the 2 channels as bubbles come out first (and these are waste area)
Make sure surface is all wetted and coax with cocktail stick into all areas on first layer
Then maybe leave to just skin over (so still tacky) and repeat or may just go in one long session?
Pop any bubbles with cocktail stick or gently blow

Intuitively feels like this should be gently poured and touched as little as possible ( I think any prodding and touching will show on the finished surface)

Also thinking even with a perfect pour the last tiny deviations (once cured) will need shimming or epoxy putty to get the last 0.05 or so dialled in?