Tips for local tours in Haarlem / Leiden / The Hague

Note that Viator is a third party broker, and they are vendoring with local services/ guides. It's better to deal directly with the actual guide.
If you see a Viator tour, look closely and you'll see with whom Viator is vendoring, then contact that guide directly.
Good luck.

".... ideally, meet us at the appropriate train station(s) and take us by car -- with some walking, no doubt -- to attractions that we want to see?"

Speaking for Haarlem, the most of the city centre is all pedestrian and doesn't allow cars (without a permit) during the day time. The train station is in the northern part of the centre, within the 17th century ramparts (the pointy bits on the map). Getting from there to the major sites will be virtually impossible by car. Sure, one can drive you to the Teylers museum, the Adriaan windmill or the Frans Halsmuseum, but it won't be a tour and more of a taxi ride so not the experience I think you will be looking for and I would not recommend this. I am afraid it will be more than "some" walking.
The Grote Markt with the Grote/Bavo church, the 14th-17th century city hall and the 1602 Vleeshal (meat market) are a 15 minute walk from the train station and cannot be reached by car during the day (especially on Saturdays with the market) and upcoming events in August. When you go through the Jansstraat, you already go past sights like the Janskerk and with a slide detour to the Walloon church and the oldest almshouse in the Netherlands on the Bakenes. From the Grote Markt, the other attractions are within 15-20 minutes walk.

Free walking yours can be downloaded from the Tourist Information website: https://www.visithaarlem.com/en/to-do/folders-en-routes/. You can also book a 90 minutes tour with a private guide through them: https://www.visithaarlem.com/en/to-do/stadswandeling/, although it looks a bit pricy at 115 euro. Alternatively, you can join the city walk, offered every Friday and Saturday at 2:00 pm. The cost is €9.50 per person and there is a maximum of 14 people per guide.

There seem to be some private tours being offered for Haarlem, but, as a local, I don't have any experience with these and cannot comment on the quality. I see some with reviews on Tripadvisor, but maybe some of the readers here could share their experiences in addition the the ones offered by the Tourist Information Haarlem.

You may want to consider the free walking tours from the Tourist Information Haarlem, so you can do the tour in your own pace, stop over for lunch or a drink when you like the sight of something and spend how long you want at any of the attractions. Haarlem is very easy to navigate on foot (and not by car).

I understand that you are looking for a tour guides, but if you don't find one, the Mauritshuis is very easy to visit on your own. Their phone app gives you commentary on a good number of the paintings. It is an impressive museum but not overwhelmingly large. The Mauristhuis website has suggestions for other nearby things to do in the Hague including other mesuems, sites, and suggested city walks. In both the Hague and Leiden, it is pretty easy to walk from the train station to the center. If walking isn't your preference, I believe there are taxis at the train stations.