Under what circumstances should I consider calling the ride off en route and leaving the riders to their own initiatives? I think that would depend on the number in the group and if any are potentially vulnerable.
Probably fairly unlikely you would have to abandon an entire ride.
I have the idea that your rides are designed to attract a mixed bag of folk - that might mean that some struggle with the distance - I/we always gently encouraged such people along, but after a point you may find that someone holds back the entire group who you might be keen to get to a lunchstop or back to down before nightfall. I usually found that folks would volunteer to leave under such circumstances so I always made sure that they were fine to get to a railway station, or accompanied them there. With other folks, though rarely, you might have to gently hint - only ever a problem once with a very peculiar passive aggressive soul who seemed to delight in freewheeling when she could clearly pedal. Luckily the north downs came to our collective help and she eventually bailed or she would probably have been found a couple of months later in a shallow grave. Very much the exception of course.
You may not have any problems with your tyres OP but with regard to punctures I would check that you are OK getting tough tyres on in case someone needs any help. But let the puncture victim try to sort themselves first. Some modern combinations of rim and tyre and can be a real pig/reduce folk to tears/be near impossible. I carry a couple of long laces to help with this.
Oh, in addition to suggesting folk check their tyres for debris before the ride, suggest that they check the sidewalls - should have done this myself (!) on a ride I once lead from London to Brighton - tyre blew not too far from mr dogtrousers, luckily I was able to borrow the bike of one of my charges and pedal home and back with a spare tyre.