Ireland and Northern Ireland itinerary help,

If you’re planning to visit Belfast, you might check out The Harrison Chambers of Distinction, a wonderful Victorian guesthouse near Queen’s College…lovely and quirky (every room is named for an author) and offers a very nice breakfast (included if you book directly if I remember correctly). There is also parking onsite. I stayed four nights last month, and highly recommend!

I don't know why you would exclude Derry on cost grounds. Compared to Dublin (especially) and Galway anywhere in Northern Ireland is reasonably priced. My impression is that Derry is better value than even Belfast.

Oh, and don't forget your UK ETA.
You still need it to cross the border, even though there are no checks.

From Galway, we drove clockwise up and around thru Northern Ireland, finishing in Dublin. Before you get that far north, is there a chance you’d consider flying the little Aer Arann plane to Inishmore in the Aran Islands? You could spend the night on Inishmore before returning to mainland Ireland, or just visit for the day, flying back and maybe staying in Clifden that night.

We then stayed on Achill Island (connected to the mainland by a bridge) rather than Westport, and then a B&B outside Donegal, then got all the way up to Dunfanaghy for a couple nights. We stayed in Bushmills for seeing the Giant’s Causeway, then down to the Belfast area, where we stayed a bit to the east, at the Old Inn in Crawfordsburn. Finished in Trim for Boyne Valley neolithic sights, before returning to Dublin.

Don't miss Donegal, the county, not necessarily the town. I loved Killybegs for its access to the Slieve League by land and sea without a car. With a car, I would stay anywhere in Donegal with a sea view.
I found Derry to be expensive and with few options for two friends traveling together and wanting a wall between us, but we didn't have a car, so we were also limited by geography, as well. Also, I found Derry to be too much of a party in the evenings, and I struggled to find a pub with a menu that didn't read like the one at Applebee's (e.g., fish tacos and pasta). Several times on the Forum, folks have recommended staying in Letterkenny, where you can leave your car and take a bus into the city for the day. There is also a parking garage at the mall, which we used on another trip when we visited Derry as a day trip from Bushmills. Derry is worth a visit for its walls and the Museum of Free Derry and the Bogside murals.
Bushmills is a great choice for visiting Giant's Causeway and nearby sites. It's a nice little town, and the distillery is just at the edge, so you don't have to drive after your tasting (if that's your thing). We stayed in an AirBnB just outside the village, but I had a pint in the courtyard at The Bushmills Inn, and it looked very nice. Google Maps shows several other places close by.
I would prioritize all of the above over Belfast. Which is not to say that Belfast isn't an interesting city to visit, only that cities aren't the reason to go to Ireland, IMO.

Be sure to tell your rental car agency that you will be traveling to Northern Ireland and to get your ETA.

Thanks to all with your helpful replies! Very appreciated!

we found Ibis coleraine reasonable, clean, etc and also conveniently located. stayed there between giants causeway/dunluce castle/coast sights and derry/grainan aleach. I wouldn't call it quaint but it could be a decent base, easy to hit grocery stores, etc as well.

definitely include donegal -- we really only managed slieve league (worth it!) with tight time but there's SO MUCH.

Thanks everyone who took the time to share their insights.