We have 334 million people in the US. Its not possible or even a good idea to make service mandatory. The Democrats under Obama tried to pass a bill similar to what you are discussing. Its foreced labor and thankfully unconstitutional. What were you thinking?
The military draft is not as you say “unconstitutional”.
What IS unconstitutional is a forced conscription (draft) targeting men only which was on February 2019,ruled to be unconstitutional.
Since women are currently exempt from registering for the draft that would have to be amended with the passing of a law by Congress.
AS for what I am thinking by my suggestion, that would be to force if necessary people to pay back this country for all they have had given them while living inside it’s borders.