Tyre out of round

If this question has been answered before, my apologies. Direction to the answer appreciated.
I use my 80 Series Landcruiser almost exclusively on bitumen to get to limestone tracks to go on the beach. Tyres are fully inflated except on sand and are re-inflated immediately on return to tracks.
Otherwise it's parked in my driveway.
The last four times I have bought tyres it was because one has gone out of round well before tread required replacement. One shot tyre means buying two.
Various brands used but not top end, expensive ones as advice from tyre people suggests no real advantage, given my use.
Originally used to deflate for tracks, but advised exaggerated flexing on deflated tyres causes ply adhesive to "let go", causing air bubbles and "out of round" problem.
I therefore adopted the practice of only deflating for sand, but the problem has arisen again.
Any thoughts?