It is true, lymph nodes are where antigens are taken to be introduced to the adaptive immune system, but the mRNA in Covid-19 can't be treated like an antigen.
Antigens are often taken into the antigen presenting cells in vacuoles that are destine for lysosomes. Antigens are then chopped up and fragments of them are displayed on the outer surface of the APC. If the vaccine mRNA was treated like this, the immune system might try to build antibodies against it, but that would be useless since the immune system won't encounter mRNA during a regular infection since it is encapsulated wtihin the virus. You need the immune system to recognize the spike protein antigen.
The goal of the vaccine is to efficiently get the transcript into the cytoplasm of muscle/stromal cells where the ribosomes there can translate them as if they were one of the cells own transcripts. That way the actual spike protein can be released into circulation. That is what you want the immune system to encounter and build antibodies against.
I hope this helps! Thanks for the great questions.