OK, why has it taken me 4 months of lockdown to realise that I've got the ideal room-scale VR space out in my garden! Having thought of the idea I did have some doubts about a) were there too few straight lines for it to manage the tracking, b) would rough grass flag as intruders in the scene and c) what happened if the dog walked through, but in the end it all worked swimmingly.
Wifi reaches about half way down - so that may be an issue, although I found it hard to draw out more the first half the garden as a space. Oculus kept putting its "draw boundary" panel right where I was looking and walking and drawing didn't help - but I'll see if I can do better another time. I ended up with a space 15 paces by 7 - far bigger than the attic (and no slopey ceilings).
The image below shows a rough mapping of the space to the WebXR demo room - so I could walk about half of it (Oculus hides the warning barriers in photos - annoying in this case as I'd set myself up to show the exact extent!)
After that everything worked just as though I was indoors - apart from the occasional need to walk back closer to the recover the wifi. I certainly lost all sense of where I was in the garden and alignment, the soft grass didn't interfere with the immersion, and the slight slope up to the house end was a useful warning!
Not related to being in the garden I did notice that I felt more latency/unease with the 3D photospheres (and even more with the stereophotospheres) than with the true 3D spaces - where I felt none at all. Perhaps one reason why there were a lot of reports of unease with VR is a lot of people were having photosphere experiences - although admittedly true latency issues remain (but made worse by doing "crazy" things in VR - like rollercoasters - rather than just walking around in a room!
One experience which was heightened was the Mozilla vertigo experience - walking on ever smaller blocks over nothing. I suppose because I could move more in the garden I could better explore it and fully immerse myself in it - and it certainly made me check I could feel grass under my feet before I stepped - particularly when I just stepped off the blocks and into space.
Anyway all the space allowed me to have a good walk around the solar system model without using teleports and actually get the planets lined up for the first time! Even in the garden they are proving too big so need to at least halve the sizes!