Re: 10-50 seat commercial passenger aircraft are a thing.
"I think screening rules differ for smaller and/or private aircraft. Also, any screening would much quicker if (a) there's 20 passengers rather than 200 and (b) there's a few dozen flights per day out of a small airfield rather than a few dozen flights per hour from a major airport"
For small operators of "flight on demand" services, screening isn't required. Any regularly scheduled flight of the public requires screening and also any aircraft over a certain size. I was aghast when a group of business people rented a 737 and visited a nearby airport (non-commercial) and the TSA had to bus a gaggle of gropers out to fondle everybody before they reboarded the plane. These were all high powered business leaders traveling together on a plane they had rented just for their trip. A state governor was also part of the party.
I would love to be able to get closer to my destinations than a commercial airport provides. In the US there are small airports all over the place. One problem is that screening requirement for regularly scheduled flights and there are regulations/permits/requirements for an airport to be able to have non-private passenger service. In many cases, it isn't worth it financially for the airport to get all of the permissions and put in all of the upgrades to make it worthwhile.
I just looked into this after seeing an add for a flight subscription service that uses 8 passenger single propeller planes for flights on demand. It's about US$2,000 month for unlimited trips and they only travel to certain airports within the state that have a Part 1XX (can't remember the number) permit.