The well respected Rabbies have a 2 day tour from Glasgow which will take you to all your places except Kirkcolm.
Maybe you could hop off when they go to Port Logan and take the bus I have referenced above. If you ask them maybe they would even bespoke one slightly for you.
Worth asking them.
https://www.rabbies.com/en/scotland-tours/from-glasgow/2-day-tours/scenic-south-west-scotland-2-day-tour
There is also a company called Strathmore Travel, who do a tour from Glasgow or Edinburgh to Portpatrick 3 times this year.
You can book this as 'self drive', joining the tour independently at Stranraer, and take part in whichever of the excursions you want. I can see very easily how you and when you could dip out of this and add Kirkcolm and Gatehouse yourself using public transit.
https://www.strathmoretravel.com/itineraries/257-portpatrick-and-dumfries-house?t3p=
Just for the half board hotel and most of the excursions it is a keen price.
Maybe not a company you'll have heard of, but I have travelled with them. It'd be a bit daft for me to Glasgow to join it, but I would willingly join it at Stranraer as "self drive", although sensibly I could easily do this or your itinerary by public transit very easily from a B and B in Stranraer. The local transit is better than other people give it credit for and very affordable at just over £6 a day for the whole of D and G.