This is making being a member meaningless because everyone already is by their implicit desire. Who doesn’t want to go to heaven?
Here is the problem. You misunderstand invincible ignorance. It does not mean that anyone who wants to be saved can be. An implicit desire for baptism will only suffice if the person is invincibly ignorant of the reality that the Catholic Church is the true Church of Christ.
I suggest reading the Catholic Encyclopedia on Ignorance:
newadvent.org/cathen/07648a.htm
I made my own outline from my past studies on the subject that I compiled from the CE:
• Invincible Ignorance:
Ignorance is said to be invincible when a person is unable to rid himself of it notwithstanding the employment of moral diligence, that is, such as under the circumstances is, morally speaking, possible and obligatory. This manifestly includes the states of inadvertence, forgetfulness, etc. Such ignorance is obviously involuntary and therefore not imputable.
o Invincible ignorance, whether of the law or of the fact, is always a valid excuse and excludes sin.
o It is undeniable that a man cannot be invincibly ignorant of the natural law, so far as its first principles are concerned, and the inferences easily drawn therefrom.
o This, however, according to the teaching of St. Thomas, is not true of those remoter conclusions, which are deducible only by a process of laborious and sometimes intricate reasoning. Of these a person may be invincibly ignorant.
• Vincible Ignorance:
Ignorance is termed vincible if it can be dispelled by the use of “moral diligence”. This certainly does not mean all possible effort; otherwise, as Ballerini naively says, we should have to have recourse to the pope in every instance. We may say, however, that the diligence requisite must be commensurate with the importance of the affair in hand, and with the capacity of the agent, in a word such as a really sensible and prudent person would use under the circumstances. Furthermore, it must be remembered that the obligation mentioned above is to be interpreted strictly and exclusively as the duty incumbent on a man to do something, the precise object of which is the acquisition of the needed knowledge.
o Types of Vincible Ignorance
• Affected:
When ignorance is deliberately aimed at and fostered, it is said to be affected, not because it is pretended, but rather because it is sought for by the agent so that he may not have to relinquish his purpose.
• Crass / Supine:
Ignorance which practically no effort is made to dispel
o Vincible ignorance, being in some way voluntary, does not permit a man to escape responsibility for the moral deformity of his deeds; he is held to be guilty and in general the more guilty in proportion as his ignorance is more voluntary.
o Hence, the essential thing to remember is that the guilt of an act performed or omitted in vincible ignorance is not to be measured by the intrinsic malice of the thing done or omitted so much as by the degree of negligence discernible in the act.
o It must not be forgotten that, although vincible ignorance leaves the culpability of a person intact, still it does make the act less voluntary than if it were done with full knowledge.
o Vincible and consequent ignorance about the duties of our state of life or the truths of faith necessary for salvation is, of course, sinful.