all travel, 24x7 access on my schedule for less than 250$/month unlimited mileage. I lower car costs by buying used and taking care of my cars so I can get every last mile of use out of them.

local is about 100 mi/week

weekend travel to isolated central and western mass (frequently without cell service). sometime need to carry 100lb of telescope kit.

1-2 x per month, make trips 4-9 hrs away from Boston and back.

I have my routes mapped out including where I take a pee break whether i need to or not.

I enjoy riding in solitude. I usually don't play radio or stream music. just the quiet of the car on the road in the deep dark of the night.

For 2 car families like myself it would likely be a 2 stage process.
Initially the service is likely to be geo-fenced so the service could replace commuter trips and visits to family and shops. We could sell the second car and reserve the first car for activities like beach trips, hiking, and holidays.
The service provider using rentals for trips outside its region might end up being much cheaper than standard rental rates because of economies of scale and the fact that the paperwork only need to happen once. In addition as you probably said, the rental might also be a robotaxi that can self deliver and then be human driven for part of the trip. The self delivery and drop off removes both a cost to the rental company and an impediment to the renter.
To lose the second car, availability of the right vehicle at the right time is going to be important, for example, I go surfing and would need a vehicle with a roofrack (for the longboard) or one with a suitable cargo area when I take only the shortboard.
I would also be happy for this to be pay as you go rather than built into the subscription.
When the service area expands to include my favourite surf locations it would probably be nice to hire the vehicle for the duration of the trip, so I can leave clothes/towel/phone in the car. I imagine this would happen naturally anyway as it might be quite hard to redeploy the taxi to serve someone else from such a location. My surf trips are usually 3hrs or so, but for whole day trips to the beach the use pattern will be similar to a daily commute, off to location x in the morning and back home in the afternoon/evening with a similar dead head miles issue.
If it turns out that the taxi can be redeployed then I might have to invest in some sort of secure storage box with combination lock I can leave at the car park with stuff I don't wish to carry to the beach.