But St Thomas finishes by saying that you can get circumcised for other reasons than fulfilling the old law. Thus many Catholics are circumcised today as cosmetic surgery, not because it is required for salvation. St Thomas indicates clearly that it is OK to do things associated with the old law as long as they are not religious ceremonies intended for one’s salvation.
I am reminded that Jesus was a Jew and his family kept the old laws. Here is a way we can understand and come close to Christ to feel what it was like to be Jew in that time, like meditating on the rosary, especially the presentation, etc. Abiding by the old laws, as a Catholic for the reason of understanding the life of Christ and being closer to one’s Jewish family seems to me to be OK according to St Thomas and the apostles.
You can have some pork ribs sometimes though, do work on the Sabbath sometimes, eat some oysters, because you know that it does not jeopardize your salvation. You were living by the old laws for other valid reasons.
I think St. Thomas would agree that being circumcised for medical reasons is morally permissable, but I also think you are misstating his poisition on the ceremonial law. You are referring to Reply to Objection 1 in your post. First, keep in mind that Thomas is giving two differing opinions, that of Jerome and that of Augustine.
Jerome holds that it is mortal sin to observe the ceremonial law after Christ. Nevertheless, the Apostles observed these ceremonies without sinning, because they observed the ceremonies only to avoid giving scandal to the Jews, and they observed the ceremonies only outwardly in that they performed the acts without the mind to observe the ceremonial law. However, Thomas dismisses this explanation, saying it would be unfitting that the Apostles obscured the Gospel to avoid causing scandal.
The explanation he endorses is Augustine’s, which proposes an intermediary period between the coming of Christ and the publication of the Gospel in which the law was dead, but it was not mortal sin to observe it. However, after this intermediary period, it would be a mortal sin to observe the ceremonies of the law.
From this, you can see how your view is not in accord with either opinion. First, with respect to Augustine, he holds that it would be mortally sinful regardless of whether you believed it necessary for salvation. That would have been a valid excuse only in the intermediary period and only for Jewish converts (" it was lawful for the Jewish converts to Christianity to observe them, provided they did not put their trust in them so as to hold them to be necessary unto salvation"). If anyone even then had held that the legal ceremonies were necessary for salvation, then they would be “fallen from grace” as Paul says.
Although it might appear at first that Jerome is saying that it is permissible to observe the legal ceremonies as long as it is kept in mind that they are not necessary for salvation, that is not his intention. Whereas, the Apostles (according to Jerome) observed the ceremonies only as outward acts, you propose following the ceremonial law as the ceremonial law, even if only for educational purposes and knowing that it is unnecessary, and you do not have the mitigating circumstances of the Apostles.
The principle behind Thomas’s reasoning is that our deeds can be a profession of faith as well as our words. Living the ceremonial law is the a profession of faith that Christ has not come. Even if you do not believe that inside, it is as if you profess with your words that Christ has not come though you believe otherwise. It is still blasphemous and potentially a cause of scandal.
Even if St. Thomas were mistaken an it were not sinful, “it’s educational” only gets you so far. It would be sinful to sacrifice animals to Jupiter even if you did not believe in Jupiter and you did not intend anything with your sacrifice and it were fir educational purposes. It is my own opinion that if what you propose is not mortally sinful, it would still be an insult to Christ for the same reason Thomas gives.