While reassuring that 0.5tph is still possible post-rationalisation, I would be quite surprised if it actually happens. Maybe if there's a surplus of trains in the 156 replacement order. Would make the railway an actually useful service, since it'd match Citylink in frequency and speed while offering a potentially* more comfortable ride (with a bit of a price markup).
*Class 156 seats + 1 transfer is less comfortable overall than a direct Citylink coach but this could change with new rolling stock
Having space for a bus interchange at a relocated Stranraer station is also great, as it'd improve connectivity to Portpatrick and the Rhins of Galloway, both of which are places which (while they do have low demand) are very cut-off from civilisation in general.
Could anything be done with Barrhill station to increase ridership? A bus to/from Newton Stewart timed to connect with trains may have some potential, as it'd give a ~2h 45 journey time to Glasgow compared to the current ~4 hour connection options. Realistically running a rail-link bus to Newton Stewart is probably a lot more feasible from Stranraer than Barrhill if there's going to be a bus interchange there anyway, but there would be about a 40 minute penalty for northbound trips which is why Barrhill would be a more convenient location overall.