What I have narrowed it down to, essentially, is the HE1000SE, Focal Clear MG, Arya Stealth Magnet.. If I purchase one of these, will I ever be truly satisfied with just the one pair? Or will I long for an experience that other pair's of headphones have to offer?
None in this sub can probably tell since every person is different with their preferences. You can be the type of person that would already close the book in your headphone journey after one big purchase, or you could be wanting to explore all the different sounds offered by the different headphone companies. If you have the option to visit a headphone store which has some of those headphones available to demo, you can give those a try and see which you'd like best.
On a personal note, out of all the above you mentioned, I really only tried the Focal Clear and they're perfectly fine headphones for all types of genre, although they have a slightly sizzling treble for certain tracks, you can do some mods or EQ them out.
There are those who are "purists" and deviating from how the headphone was tuned by the manufacturer is going against the point of listening to the headphone they created. That said, I'm not one that finds the harman tuning cater to my preference so I adjust my EQ differently than what Auto EQ has recommended. However, AutoEQ does have the baseline starting point that I work with when using EQ.