The constraint on public spending is the amount of real resources that can be utilized. Productive utilization of resources is not inflationary. For example, if you have 10% unemployment, a government can hire those people to build infrastructure regardless of the debt ratio. On the other hand, if you issue public debt to purchase commodities that’s obviously inflationary.

But in either case, the constraint is real resources and inflation, not the debt ratio.

You asked why countries haven’t done this already? They have, it’s called quantitative easing and they do it in financial crises to absorb the productive capacity that the private market isn’t utilizing anymore.