I think "positive nihilism", which could describe my own beliefs, is not a restrictive term and simply means that one sees no inherent meaning but is for whatever reason not bothered by it.
Nihilism doesn't necessarily lead to being humbled, I think stereotypically that an individual's nihilism is not nearly as nihilistic as it could be. Take time as an example, nihilism means that time is inherently meaningless, our feelings about it are subjective, based on nature/nurture influences, as well as our choices. There is nothing inherently good or bad about my life being so short in comparison to eternity but the individual nihilist cannot shed human psychology with just thoughts. What do we feel about this? Anger at the unfairness? Are we jealous? Insecure about our relative impact? Scared of death? These feelings lack inherent meaning too but saying that doesn't help.
Mostly, existential nihilism is not a result of the logic of nihilism but the framing of nihilism, which can become the backdrop for all that we are, everything we do, everything we think, it's all meaningless. Believe this and think it often enough and it can cause anxiety, stress, depression and all that we describe as existential nihilism. Yet again, this in itself is not very nihilistic, shouldn't it be true that both meaning and meaninglessness lack inherent meaning under nihilism? Nihilism itself is an argument which uses meaning, if "proof of nihilism" doesn't mean "there is nihilism" then where does the belief of "there is nihilism" come from? Meaning is taken as fact and has repercussions for how one lives, thinks and feels.
The distinction of inherent meaning and asserted meaning is made meaningless under nihilism. Within nihilism, all meaning is asserted by the intellect, including what is inherent or asserted. There is nothing which is exempt, every argument can be waved off easily, worst-case scenario, theoretically at least, one could abandon logic and reason. Nihilism says there's no inherent meaning to that and I could reject the meaning one might think it has. In much the same way that God dictates within a religious scheme, the intellect dictates within nihilism, God's power has been taken away. Even if there were a God, his "Godness" has no inherent implications for the tyranny of the intellect, to assert or reject meaning at will. Neither time, mass or space, or even the divine, can contend with this power to dictate meaning.
Yet we are restricted, can't I only interpret whatever I wish if I am mad? I need to work within my limitations, I need to convince myself or it's pointless. The nihilist deals with two restrictions: Convincing themselves and contending with that which can not be changed. The two ways around a problem, firstly, for instance, with time, I can neither be immortal nor pretend like I'm indifferent to whether I live or die. Those are my limitations. In the end, it all comes back to human psychology, or my psychology and my Earthly circumstances.
Therefore if the issue of eternity can only be dealt with by either convincing myself of an interpretation of my mortality that allows me to accept it or by becoming immortal, then without the divine, it's got to be the first option. The second option becomes available through religion, rebirth or an eternal afterlife, it means you can take the second option of simply becoming immortal. The first option is not unachievable though, there are logically consistent and believable ways to come to grips with your mortality.
The intellect "finds" meaning through asserting meaning where neither restriction applies, patterns are the result of similarities in our nature/nurture circumstances that play a large role in what the restrictions are. Under nihilism, things can be split into meaning which the intellect dictates and meaning which the intellect is forced to accept by their intelligence. And when I say intelligence, I don't just mean that in a positive way, for instance, to deal with something about which I might be jealous, I need to deal with the cause of my jealousy, simply speaking words is not enough. Intelligence is both conscious and unconscious here and also, it can't be divorced from my biology.
I think a major issue then for belief in God as a result of pragmatism is that you're intentionally being intellectually dishonest in a way which shouldn't be able to fool you. Even if believing in God was pragmatic in that it provided you with psychological benefits of being happier or more fulfilled, you couldn't actually believe there is an eternal afterlife just because you think it'd be better if there were one and you can't become immortal either. One might be able to convince themselves of something based on a belief that the belief is pragmatic but there are rules for this, usually an element of truth is required. In this case, with an eternal afterlife, from an atheistic perspective, there's absolutely nothing to work with at all and it's pretty much hopeless.
I think that while I disagree with many of your points, this is the main issue. However, I would also say that the optimistic nihilist within these two aforementioned restrictions has an incomprehensible range of options. I think the problem of mortality can be dealt with before we need to rely on simply getting rid of it by becoming immortal. Mostly statements like "we are just mortals" need to be contended with, actually, mortals are the only source of intelligence and can only be contested by other mortals on intellectual matters. "We are only mortals" in philosophy, it's almost a nonsense thing to say when you're an atheist and with nihilism, mortal or immortal, mortal or divine, there aren't even hard and fast rules about what those things mean, certainly nothing an intellect can't rewrite.
Optimistic nihilism should work within these two restrictions, recognising that equally true or equally not untrue interpretations can be chosen between by their pragmatic benefits and aim at creating a worldview which produces positive effects. Your proposal here simply fails to work within these restrictions and probably isn't the best or easiest method by which one can view their mortality and think of it in a way which brings about desirable effects.