Thanks for all comments so far.
Using 18mm MDF.
The sheet being used isn't full width, so I still have the full length offcut to use as a straight edge, with clamps.
Haven't quite worked out door size yet, almost to the ceiling, but doesn't need to come down to the floor though as the bottom part is a boxed in stairwell which will be obscured behind the bottom part of the panel.
So the door needs cutting out on all 4 sides, there will be plenty of 'meat' left in the frame, all the way round.
Softwood frame (2x1 & 2x2) is affixed to walls, floor and ceiling, plenty rigid.
I can unlock the depth adjustment in the saw and static plunge to get the hole started.
I haven't used router for a couple of years, I forgot about the multiple pass technique. Sounds too messy though, and all my worst mistakes in the past have been with a router!
Out of interest, has anyone ever used the Kreg hinge cutting jig? £35ish, looks useful.
I have a homemade one I made years ago, but was a bit keen and went through a door that I'd spent ages working on!