I don't know that I have any issue with what you've said, except you should continue on to quote v. 17.
And, to note the context..throughout the Gentile church, Judaizers were attempting to enforce the Mosaic observances, including the festivals, upon Gentiles. Gentiles are not Israel, and the festivals were given to ancient Israel. They were "separation commandments" which were meant to preserve Israel's distinct identity, and to prohibit idolatry.
By stopping at v. 17, you are obscuring the fact that the days (weekly, monthly, annually) are shadows of Christ, and not the substance. The substance is Christ.
Judaizers worship the shadow, and not the substance.
Some also have this idea that Gentile slaves could simply start keeping the Sabbath, festivals, and New Moons..as if their masters are going to allow them to do this. Some also think that their pathetic observances, which required going to Jerusalem and could not be properly observed elsewhere, meet the requirements laid down in Scripture (Deut 16:16). Some have told me that going to a hotel room for seven/eight days, or going on a camping trip with their kids for seven/eight days, is keeping the festivals. Wrong. They can do it if they want, and it may even teach them some lessons, but it is not true observance of the festivals as defined in Scripture.
It's really impossible to observe the Mosaic Law now, as the Temple has been destroyed and there is no Levitical priesthood. Additionally, Christ has been sacrificed, and he is the final sacrifice. Therefore, there is no way that the Law can be observed now.
And, this is where I have a massive issue with dispensationalists, as they believe a Temple will be restored, and that the Law will be instituted again in the Millennium. Their understanding of the Prophets leads them to this conclusion, but this cannot be as Christ is the final sacrifice. And, their lame excuse of "these are memorials" is false, and violates their OWN "literal" interpretation, as nowhere does it call these observances in the Prophets as memorials. It refers to them as "sin offerings" in fact.
Instead, if one understand that the prophets were given visions that used "old covenant lenses" to explain "new covenant concepts" things become much clearer. But, the dispensationalist will claim this is "allegorizing" or "spiritualizing".
However, I will ask them if it is sane to think that animal sacrifices will be offered in the future as legitimate expressions of worship to God. I don't think so.