You completely misconstrued the meaning of the CDC advice you are quoting as an argument for your position. The point of this advice was to avoid mixing-and-matching vaccines in a two dose regime (because there wasn't much data on the relative effectiveness of such a mixed approach vs the known results from the recommended two dose shot of the same vaccine). The CDC was not saying you shouldn't take another vaccine after completing a standard two dose regime. The word "interchangeable" in this advice is key to proper understanding of its meaning.
Although, on a separate note, depending on vaccine availability in the area where someone resides, it might be selfish to take another vaccine shortly after becoming fully vaccinated.