I understand the cost constraints and the imperatives but your "whatever yard can be converted to build appropriate ships at the time" strikes me as a COA dependent on a lot of hope.
Hope that there will be yards. Hope that they will have skills. Hope that they will have done this before. I believe that at least some budget should be spent developing prototypes and low volume production that can be the basis for rapid expansion.
One of the interesting points with respect to creating yards that can pump out OSVs in peace time for the government is that they not only serve as arsenal and sensor platforms in wartime, they will also act as logistics vessels in peace and war, handy for disaster response and .....
Can be sold off to the civilian sector after 5 to 10 years of service at discount prices.
Riley Clair aka USS Nomad.
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Optionally manned, reserve crew, regular crew, coast guard or civilian.
To convert them to "warships", or any other duty, twistlock containers to them.