So much to say, but you probably anticipated that, so let me start at the top. I believe the weight tolerated is very dependent upon balance or the ability to balance your breast weight over your hips. For anyone who has carried a toddler or a bag or cement, this should be clear; you have a natural tendency to carry the weight high on your chest and never at waist level. I would suggest that 50lbs of hanging breasts are more difficult that 100lbs of breasts carried high simply because posture does not mitigate the load. We have witnessed Jenny struggle with a stated breast weight of 93lbs, but Beshine rather gracefully walk around with something probably close to that. The difference is where the focus of the weight is located, which allows Beshine to better carry the weight by her overly upright posture that equals stress between the back and her abs. Clearly, her more developed legs shows she is far more adept at physical activity than Jenny.
So where that leaves you is solely dependent upon whether you can balance your breast weight over your hips or if it is always pulling your torso forward. One thing for certain, past a certain weight, trying to lift hanging breasts higher just puts an unbearable burden on the shoulders.
Where do we draw the line? Immobility is not a goal, however, experiencing degrees of physical challenge may actually be exciting. As I have often stated, the higher held breasts may offer far more simply because of hindering arm movements, all of which may be wonderful to experience and easy to overcome without undue burden. Low hanging breasts do far less to hinder arm movements, even while sitting. The one thing you want to maintain is mobility; lying about cherishing your own size is a poor substitute for the appreciation or awe experienced when walking around in public. When you are supporting 80lbs of breasts or more, just the simple acts of sitting and standing daily will produce legs like those of a mountain biker; anterior and posterior muscles prominently developed, which is precisely what we see in Beshine. So I think our line must be at what level of weight you feel comfortable living with daily.
I think all the things you list, where your boobs dictate how things must be done, are things to actually be cherished if you really love having huge boobs. When your boob size creates the need for adapting, there should be a smug degree self-satisfaction each time you experience each, the constant affirmation that "my boobs are so big that I have to" affect. Whether it is that they take up extra space or create movements no normal person makes, it should produce a good feeling. This is why I like the high aspect; when you can no longer reach around your own boobs, your arms have to reach over them when both hands are required. When one hand is required, you turn your shoulders to move your boobs out of the way. It gets more interesting when working at a counter, because your boob height may be sufficient that you cannot actually touch the counter with your arms resting atop your boobs, which would make reaching into a sink or just reaching the faucet a turn and one hand reach moment. You mention a tray for eating; the primary concern with eating is to have your mouth over the place, and with high, round boobs, I think your cleavage would hold a place quite well.
Not being able to drive, one may assume your position is the passenger seat, however, if you have massive sideboobs, those seat bolsters will always be pressing into your back boobs, so you would have to recline the seat just to sit upright. In all likelihood, it will be much more comfortable on a bench seat. Again, you will need help with the seatbelt simply because, although you placed the shoulder strap behind you because it had no hope of ever going over your boobs, you cannot pass the belt from your left hand to your right under your boobs (again, massive sideboobs), so someone has to do it for you. Of course, that someone already loves his role.
I think the physical devices, like handrails, is more necessary with a lower center of balance. That said, as you may already know, leaning forward to stand or reach something on the floor while seated often requires moving your boobs, especially so if the seat is low. The strategy of having one boob between your legs and the other outside a thigh allows for leaning forward and the larger they become, the more the need is apparent.
This is not to say that given boobs like Beshine, but let us say a bit bigger still, you would not need help. You would not even be able to wash certain areas of your own boobs in the shower simply because your arms cannot reach them. You would likely not be able to touch your own navel, hence you would need help fastening your jeans. And while you can somewhat blindly slip your feet into sandals, someone would have to tie your shoes. While you may elect for the tent like tops, if you want a top that fits in the waist, it would have to be pulled up, feet first and over the hips (precisely like Beshine's bras).
When you lie down in bed, with hanging boobs, you can lie on your side and place them in front of you, however, Beshine like boobs are not so forgiving. Your massive sideboobs that prevent your arms from simply reaching forward, now also prevent you from turning on your side. You could just lie on your back, however, by just placing half a pillow behind a shoulder, it would turn you so that one sideboob is now lying on the bed, along with perhaps half its weight. No tossing and turning, you are locked into the position. Of course, considering your boob weight, sitting up is no longer possible, so any movement to get in or out of bed must employ your arms to push yourself into a reclining position and wiggling your butt into position. I'm sure your wouldn't mind as you watch your boobs heave back and forth while doing so.
So the bottom line is, mobility depends upon the ability to center your boob weight over your hips and, if you want to experience vastly more big boob moments, ones that inhibit reach just add to the enjoyment, because they are just going to constantly press against things as you strive to reach.