As you are seeing intuitively, he is still quite content to be a part of your family. He is very dedicated and very loyal, and very much likes being around you and your energy and is very keen on contributing his healing to the cause. He reaches out to all who are present, in fact, and works on each one as he notices things that are out of alignment in some way or another. This is serving a high purpose for him, for that is his mission after all, and as long as that is so, he is quite happy to remain here. If he were suffering from pain that would be a different matter, but this is not the case, so his infirmities are a restriction and do cause a reduction in his experience, but he has other things that stimulate and challenge him, primarily in coping with being in a sightless world. This creates new opportunities to engage him and give him challenges to explore and overcome. This is a kind of variety of his experience and not the negative it would be for a human being who knows intellectually what they are missing and will be very unhappy with the drastic reduction in a major sense like vision. This is less true of animals because they do not think about their plight and personalize it in the same way. They just react to whatever comes their way and it becomes sort of the new standard and they cope quite beautifully with adverse conditions because of this flexibility.
The issue of the burden he represents is a personal perspective and consideration to make an assessment of the relative cost and benefits here. Your love for him is strong and genuine, and the desire to help him be happy is divine. This does not mean you have the burden forever to put up with all possible negatives that may result from his presence. When he is unable to regulate his bodily functions and is in danger of destroying your property, that is over the line of necessity or a karmic balance in things. After all, he is here to give love and to be of service. If he is distracting and detracting from your quality of life because of his infirmities, he would be quite happy to leave and avoid being a burden. He does not perceive this directly at the moment when these accidents happen, so he cannot make the choice himself. It will fall to you to decide when you have had enough and you feel it is time to let go of him. At that point, you can be sure he will greatly enjoy being back in the light and the surrender of him and letting him go will be a time of great joyous celebration because his life is a sacrifice entirely. It is much more enjoyable to be in the light, and coming into the earth plane is a net negative, just as it is for the human. The animals know better what they are missing, but they do have the ability to go to the higher astral plane during sleep which humans cannot. Nonetheless, when it is their time, to return fully is a wonderful release and a significant improvement in their circumstances.
So your instinct to let him go is not a selfish act, it is recognizing intuitively, it is better to be in the light than to be here. And that is true for him, even though his soul is being fulfilled in carrying out his service and he is content and happy to do so, that is not the same as being in a state of bliss within the light full time, and that you could give to him by letting him go as your pet. So what we are saying here is that there need be no concern about feeling selfish in euthanizing your pet because of the faulty housebreaking, which seems a minor annoyance compared to the death of a beloved companion. But this is not truly what this represents. It would not be causing harm and pain and loss solely to prevent human inconvenience, it would be giving him a great gift through his release from duty. And so your perspective is quite accurate here, that he can be released at any time. There is no rush because he does not suffer and will have plenty of time in the light once he returns, so the days, weeks, months, or even years added onto his duty are not significant in the big picture of things. But you will be sensitive to his comfort and that is the major consideration here. He can tolerate his service quite well and will feel fulfilled in carrying it out on an ongoing basis, so you will most likely have to weigh in at some point and choose an endpoint. It will be a blessing for both human and animal when that happens despite the loss of contact and the grief you will feel in losing him.