Electrifying to Oxford allows for an end to the split of the locals in didcot, so improves the railway product in the area
As stated in the message you quoted, some of the locals on Cardiff-Bristol-beyond are already bimodes. The big advantage of Bristol via Bath is for the london trains, and it would be madness to not electrify Bristol TM - Bristol Parkway if electrifying the route via Bath.
Though I fully agree the routes into the southwest should also be electrified, and the heavy stone trains should be high up the priority list.
So that would give something like this for a rolling southwest/GWR electrification
1. Bristol TM (via Bath and via Bristol Parkway)
2. Oxford (when Oxford remodelling is finished, so possible after some of the later ones)
3. Newbury-Westbury-Taunton-Exeter, including branches to the quarries
4. Bristol-Taunton, including weston-super-mare and other branches around Bristol (Severn Beach, Bath/Chippenham to Westbury)
5. Working onwards along the mainline to Penzance
6. Possibly branches around Exeter?