Interesting questions! I think this is the first time they have been asked. Yes, I have corresponded with and sometimes even worked with Sisters who felt they were called to life as hermits, though my personal experience in this work is relatively limited. I don't know that former religious make "better hermits" under c 603 than those without any experience of religious life, but of those I have worked with, the foundation supplied by initial formation in community seems to be almost indispensable. That "almost" is important because canon 603 does not explicitly require one have been in religious life prior to entering into a mutual discernment process with one's diocese re canon 603. Still, when I met and introduced myself as a diocesan hermit to Bishop Remi de Roo who was responsible for the intervention at Vatican II that eventuated in c 603 in the revised Code of Canon Law, the first question he asked me was whether I had come to eremitical life from (cenobitical) religious life. (Bishop De Roo had been the bishop protector of about a dozen monks who had left long histories in monastic life. This experience led him to make the intervention at the 2nd Vatican Council regarding recognizing eremitical life in the consecrated state. He certainly understood the importance of a strong background in religious life.)
The equivalent of such a background can be gotten apart from religious life, but it takes dedicated work on the part of the candidate with the assistance of others to do that. Moreover, the time initial formation takes before first vows is correspondingly longer for those who have no background in religious life. Moving from no background in religious life to truly discerning a call to eremitical solitude and preparing for a life commitment in this state of life is a long process and while the lion's share of this formation occurs in the hermitage alone with God, it must still be supervised and directed. This requires not only spiritual directors, but mentors as well. One key to discerning readiness for such a commitment is the canon's requirement that one write a Rule of Life. Many candidates fail in this specific requirement because the degree of experience and learning it requires is very significant indeed. Such failures seem to happen more frequently in those without a background in religious life.I should also point out that those who have lived religious life before discovering a call to eremitical solitude must let go of many things too. Sometimes they also have still to develop the strong spirituality you refer to, not least because their life of prayer is no longer mainly communal. The individual schedule that works best for them may not yet be known and require experimentation over time. For instance, it might take them some time to discover that they pray best in the middle of the night or that they are late night people rather than early morning people (or vice versa).
The freedom to set one's own schedule can be quite challenging. Learning that one does not have to be active at all times, that "praying always" does not mean merely saying prayers and might (at least superficially) look like "wasting time" in the eremitical life. Finding what resources one needs for praying Office alone will take experimentation and sustaining a sense of God's presence in the absence of faithful brothers and sisters may require extra attention. Finding ways to become truly self-supporting, as well as locating a good place to live in the silence of solitude is essential and takes time and sometimes special training and corresponding resources. Too, the hermit will need to learn new ways of understanding and living the evangelical counsels that living alone and being self-supporting requires. Religious poverty will differ in some significant ways as will religious obedience. Also, dealing with personal illness, whether acute or chronic, will be different than doing that in community. For some, these transitions are very difficult to negotiate and so is the underlying freedom and sense that it all depends on their own discernment.The point is that whether one comes to eremitical life from a long period in religious life or from some other background, there will be a significant learning curve that occurs over a similarly significant period. Every consecrated solitary hermit's life is similar to every other hermit's life in major ways (that is, they all include the central elements of canon 603) and yet, it seems to me that what is more striking is the variation each life reveals. I know some diocesan hermits as well as several hermits who are exploring c 603 and preparing for profession and/or consecration. Each one embraces and lives the central elements of the canon differently than the others because their life with God differs from that of other hermits. I think it is important that anyone working with candidates for profession under canon 603 understands this and that they let go of any sense that schedules, prayer practices, etc, must look the same from hermit to hermit. (Chancery personnel have as much to learn about hermit vocations as anyone else!)It remains critical to understand that while c 603 does not formally or explicitly require a background in religious life, even so, writing a Rule of Life suitable to the living of such a life requires significant experience and formation that is more usually gotten in religious life. It can be done, yes, but I personally believe it is a relatively rare candidate for profession that will be able to accomplish this particular requirement without a background in religious life. Ironically, one of the best ways to determine the formation a person has and also still needs is by reading their Rule, or working with them as they write it. Thus, again, while c 603 does not explicitly require a background in religious life, it may be the case that the authors of the canon strongly implied the necessity of such a background by making the hermit responsible for writing their own Rule.