There were lots of different models of the old pens. The Lifetime Balance has many different models. the Snorkels all came in various widths and lengths (numerous models too). I think this is because you do not put fountain pens in pockets. Therefore, the pen needs to fit the space it will ride in - like a front pocket. I have some snorkel Sovereign and Clipper. They have 24kt nibs and write as well as anything. They were high-end pens though.
While the nibs can be “adjusted”, I have found the Parker 51’s to be very consistently smooth whether they are aerometric or vacumatic. The line-widths will vary, but unless someone has ground the nib for a left-handed calligraphy writer, they seem to write consistently for me. Obviously, a double-jewel 51 will write considerably better than a regular 51 but that is why they cost more. The various filling mechanisms are a pain. Do not get me started on the vacumatic. I hate vacumatics.
If you have not tried a Scheaffer Pen For Men (PFM) then you need to do so before writing off old pens (yes pun intended). The PFM is widely considered the gold-standard of old, production pens. It is by far, my favorite old pen. Even better than the 823. They run around $250+ on ebay though. If you get a good one; you will instantly understand why people love them so much. Just a joy to write with.
As for MontBlancs - the modern pens do have a brass filling mechanism. Those are heavy in the hand. That is why pre-1990’s MontBlancs sell for more money on ebay. They had plastic (ok, resin) barrels and are not so heavy. The modern ones are all for show. The older ones are for writing 