I think I have answered this question in the past (cf., Spiritual direction as accompaniment 2014 different title in original) but perhaps I can say something new about it here. My basic answer is that in my own estimation, no, it is not the case that we need spiritual direction less and less over time as we grow in our vocations. I believe this is a kind of temptation specific to those who are more or less "arrogant," ignorant of, or perhaps just complacent about their degree of spiritual maturity, or their "possession" of a given vocation. It also betrays a mistaken notion of living a vocation as ceasing to need ongoing formation which, while it differs in kind from initial formation it is still absolutely essential -- itself requiring good spiritual direction -- even if this direction is less frequent than formerly.
We cannot see where we have not grown, or rather perhaps, we cannot necessarily see where or how we are called to grow in a given situation. We may know in a general sense that we are called to holiness or union with God, or a better or deeper relationship with Jesus, but also not know how specifically such holiness, union, or relationship actually looks or how we take the next steps toward this, especially when we are speaking about a deepening of one's relationship with God in Christ and the personal work needed to be entirely open to this. In such a case, it is my experience that far from becoming less important and necessary for living one's vocation, it actually becomes more and more important that we are able to talk about it with someone skilled in spiritual direction who has a vital relationship with God and knows us well.
In some ways it reminds me a bit of the kind of coaching or masterclasses required by a musician who truly takes her vocation as a musician -- an artist -- seriously. We can always play the way we have played for many years, improving gradually simply by learning the music in front of us. In orchestra, for instance, that always stretches a musician to some extent, but when you watch a master musician working with someone, the experience is inspirational. One watches the master teacher bring out in that person places where they specifically can transcend their own levels of achievement and realize an almost unimagined potential within themselves (and within the music they have been playing almost routinely for many years).
I have spoken before about a spiritual director as an accompanist and the special gifts and skills required in such a relationship. I noted that it was important that the director be a person of prayer and be under spiritual direction themselves. In other words, it is important that a spiritual director know herself well, know God in Christ well, and know the person she directs well. When all of these elements come together in spiritual direction the director can accompany her client in a way which allows for steps the directee would never have taken on her own. It will be because the director knows (in at least a general sense) and trusts the potential within her directee, and because she knows what God offers in terms of an ever-deepening relationship, and also because she knows the kinds of steps one needs to negotiate to grow in this way, because really, in good directors, all three of these will be true. But, as in work with a master musician, the spiritual director sees all of these elements in a way the directee well may not and assists her directee to take the steps needed to come to the abundant life God offers and wills for her.
Again, we truly cannot see ourselves well enough to achieve the growth and conversion God wills for us, nor, perhaps, do we know God (or allow God to know us) well-enough. We need people who have travelled the same road before us who also know us and our unique capacities, potentialities, and also the various obstacles to growth/conversion which are part of our personal "obstacles" to seeing and growing. Over time we may become complacent with the degree of growth or conversion that obtains without spiritual direction. This can happen because our directors are not particularly skilled or committed to their own growth-with-direction, because we cannot find a good director, because we fail to trust our directors (or to entrust ourselves to their wisdom and expertise when that is real), and it can certainly happen when we become comfortable in our own life, career, or vocation and treat that comfortableness as though we have "arrived" in some sense. With hermits it sometimes occurs because one mistakenly believes a director must also be a hermit or a mystic, or that they must experience the same kinds of exceptional things in prayer that we do (if, in fact, these do) --- which allows the hermit to dismiss the ways in which the director actually challenges and blesses or could challenge and bless them. In such cases the hermit will say they have no access to a good director and go it alone.
You ask if it might be more likely that a hermit who is also a spiritual director would eventually not need to work with a spiritual director. In this too I would say no, it is actually less likely true precisely because such a hermit would know well the importance of accompaniment (and the danger of relative complacency, false humility, or "arrogance") in genuine ongoing conversion. One final note, there is a now-abandoned blog by someone calling themselves a consecrated Catholic hermit under private vows you might have read posts re the position you describe. Please know no one becomes a consecrated Catholic Hermit with private vows. They are a dedicated lay hermit, but neither consecrated nor a Catholic Hermit.
N.B., for a particular vivid illustration of the way a master teacher/coach can work with a skilled musician check out Zander Interpretations Class. (Benjamin Zander, Interpretations class, Gabriel Faure's Elegy for cello. Alan Todas Ambaras, cello) The initial presentation of the piece is wonderful and any cellist would be really pleased with it, but the work Zander does with the "student" (and too, the accompanist!) is truly inspiring, and startlingly transformational in several significant ways.
Spiritual direction requires the same kind of depth of seeing or vision, confidence (especially in God and the person being directed), and capacity to accompany another in a way which draws the very best -- indeed, the most true and real -- from the musician (hermit), instrument (prayer or relationship with God), and the music itself (the life of God offered to and dwelling deep within the hermit coming to fulness of life in God). Zander notes that his power as a conductor is about empowering his musicians to play "well" (and here he means helping his players tap into the power, life, and truth within themselves); otherwise he is powerless to make music or bring the music to life -- for as a conductor he does not make a sound. So too the job of the spiritual director to help their client get in touch with the life of God within and around themselves. No one outgrows this need to be helped (accompanied) in this very special form of hearing, understanding, embracing, and expressing what one hears and understands with regard to oneself and the very Life of God.
[Benjamin Zander is a noted conductor and inspirational speaker, the founder and conductor of the Boston Philharmonic and the Boston Philharmonic Youth Orchestra. Zander began his career as a very gifted composer and cellist who studied with Gaspar Cassado.]