Martino writes:

I do not want to oversize my A/C unit but I do not want to significantly
undersize it either. What do you think?

It's like politics and elections. The issues are complicated, but all that
complexity reduces to 2 or 3 choices. (Although unlike politicians, one is
usually happy with a choice of air conditioning system.) If your hunch
between one or the other choice is usually right, then it may not be worth
performing a costly analysis. And as your calculations show, the results
depend on some noisy assumptions, so your results are no less noisy than
that, and your hunch may be just as well grounded, especially if you do a
lot of installations, and extra especially if you're replacing an existing
system of known capacity and knew how that performed.

While the Manual J process is sound in physical principles, it also serves
a marketing role. A clever salesman can manipulate assumptions and other
inputs to yield the result he prefers.

Another marketing role was that only "experts" could do it, and any other
competitor was therefore a "hack", but that is becoming less of a working
distinction as the software and publications become available freely on the
Internet, as they should be.