Thanks again for all the responses! At this point in time, I have decided to set fretless aside, at least temporarily.
While the rational/analytical part of me finds very little reason to continue, I find that I do not want to totally quit and sell my fretless.
In terms of "cost" vs. benefit, the only real benefit (for me personally) from fretless is a pleasing tonal character. While that is a benefit, this particular "pleasing tonal character" is something that I can certainly live without. There are other bass tones that I dearly love as well. The "cost" in terms of difficulty and stress do not seem even remotely worth the benefit to me, from where I stand at this point. (Could change down the road, we shall see.)
As stated earlier, I don't do slides or vibrato. I don't have anything against these techniques, and I appreciate them when they are done well by others, but they are just not my thing, personally. I have always tried to play fretless pretty much exactly like I would a fretted bass, in this regard. I suspect that the ability to use these techniques is one major reason why many people choose fretless, but it is a "non-reason" for me.
Upon further reflection, I have come to realize that the main reason I was playing fretless was really to have a way to compensate for out of tune music. Other than practicing fretless for the sake of practicing, the only thing I was "using" it for was tracks that were not in agreement with the rest of the (440) world. Case in point: I was given some original tunes (rough recordings) to create bass lines for. Since they were a little off in terms of pitch, I just grabbed the fretless, and composed the bass lines on it. If these tracks had been in tune, I most certainly would have just used a fretted bass.
Based on this thread (and the other one I mentioned earlier in this one) I am convinced that this was a very bad idea on my part, and a large part of the reason why I struggled so much with intonation. I have since learned to make pitch adjustments in my DAW, so that motivation for fretless use is gone now anyway, in addition to me realizing it was bad idea. Case in point: I have been wanting to learn the bass line to "Blood and Roses" by the Smithereens, but that track is off on pitch just a little. As such, I had been trying to learn it on the fretless. However, I recently used my DAW to make a pitch adjusted version of the original recording, and have since mostly figured out the bass line on fretted bass (I have the main idea down, but not quite there yet in terms of "note for note" on the little nuances.)
Furthermore, where I am at life right now, I simply do not have the time available to practice as much as I would need to in order to really do fretless justice. Given enough time and practice (especially if I were to start practicing only with "in tune" material), I have no doubt that I could eventually get it figured out. Between work and family, it is not uncommon that I will go almost a week without having a spare second to pick up a bass at all. I fully believe that my only hope of getting anywhere at all with fretless would require daily practice, and that is simply just not possible right now.
Those are my main thoughts for now, and again I offer thanks for all of the goods thoughts and suggestions!