I do not know if you aware or not, but there is a similar to mRNA vaccines, yet different (actually, almost mirror reflection of what mRNA vaccine is) invention called antisense drugs. Those are actually already used as antivirals.
Specifically, mRNA vaccines require an immune response to be effective. The mRNA (vaccine) encodes a viral protein, and the host (human) immune system still has to learn to respond to that protein.
Antisense drugs are much simpler, with no immune response required. The mRNA (the drug) itself simply binds to the target mRNA.
Interesting re: the thiophosphate group. It sounds like it just floods your body with some mRNAs complementary to the viral mRNA that blocks it from getting turned into proteins by your cells' ribosomes?
Does that make them a class of "cures" rather than "immunizations"? If I get infected with the same thing a year later, will I need another dose of the antisense drug?