Thanks, Im. I know you have all-too-recent experience with also losing a close 4-footed friend. Not even a year ago yet. I suspect that the loss of Ajax is still painful for you.
I am not sure yet when it will happen with Lucas, but given that he's having difficulty even lying down long enough to get his belly rubbed (apparently it is more pressure on his open lung when lying on his side, or else it starts to fill if he's not upright), I don't imagine it will be long before he communicates he's ready to go.
Luckily I asked the vet something that a friend had mentioned was done for her cat. And I was very happy to find that the vet is willing to make a house call to put Lucas to sleep, so that his last moments are not spent stressed in a car and unfamiliar place, but are as comfortable as they can be in the home he's lived in for over 10 years now. It felt much better knowing that I wouldn't have to subject him to that stress, given how much he dislikes car rides and the carrier. [Not that many cats like either. I think I'm kind of rambling I'm so far past capacity here.]
Thank you again, Imrahil, goflyblind and MartyS, for the kind thoughts.