Looks good, although I was thinking that rather than the bracing I would
have two or three legs on the other side of the plinth going down to a
joist that spans a supporting wall.

That's probably safest. I'd be very wary of supporting close on 150kg at the
top of a (usually badly built) firebreak wall in a loft space. The loading
has a large sideways element in it, too. If it was straight down, it would
probably give the wall greater stability.

Christian.