What strikes me (and not to step on toes Father - of course you'd know better) ...
But what strikes me is that if we closely observe souls and how they are affected, is the realization that there is a dynamic between a person, and sin, and I'm not sure what all else.
What I mean is that at a certain point, a person can commit a particular sin. I'll use what many would consider a mild example. Say they take home a pen from work - a very "venial" sin from a Catholic definition. It might be that such a sin barely deserves remembering, so is not really repented of, and is not mentioned in confession or even in personal prayers. And the person is not seriously damaged by that sin.
(In fact, we might easily commit a number of sins of this degree and not be truly affected by them as individual sins - though I think a pattern of such "minor" sins and accepting them as minor could potentially lead to hardening oneself and potentially moving away from repentance.)
But ... my point is that the same person at a different point spiritually could commit the same sin - purposely and willingly choosing to steal a small item - and this can represent a HUGE disobedience against God and a rejection of Him - at least temporarily. It can be a very damaging sin, one that has an effect on the souls of "mortal sin" proportions. The actual sin might be the same, but because of differences in the person they are affected differently, and they desperately need to repent.
I hope I'm explaining this well enough. My point is that while I admire the intention behind the effort of Catholicism to make a distinction between serious and lesser sins, in fact it CAN depend on the person.
Some of the Saints who enjoy the ongoing grace of God fear even allowing the first dart of the enemy in a suggestion of sin to the thoughts, because they know how likely they might lose that experience of Grace simply for allowing that thought to reach the point of being recognized, which of course happens continually to almost all people and isn't even truly sin for them.
And between the extremes we can be at various points. So I think there can be errors made if we assume this sin or that always has a given magnitude.
Just thinking through this a bit.