You can subdivide the mesh while dynamesh mode is active and won’t lose the resulting subd levels when you decide to turn dynamesh off. ZBrush will bug you if you already have subd levels prior to turning dynamesh on though, in which case you can either freeze them or delete them before enabling dynamesh (I would not recommend subdividing a dynamesh with frozen subd levels though).
Might be worth noting that InsertMesh brushes don’t work if you have subd levels, so if you plan on doing an boolean style stuff you’ll want to keep that in mind. A better choice would be to use the “DynaMesh Resolution” slider to control polycount when remeshing. The general methodology is to do as much work as you possibly can at your lowest resolution before taking it up a notch. Once you’re happy with the general sculpt, the idea is to turn dynamesh off for doing the final super fine high res details (pores and wrinkles for example).
The idea behind dynamesh is for blocking out the general shape of what you plan on sculpting. You can work at high polycounts with dynamesh on if you really want to, but you might lose details whenever remeshing. Experiment to get an idea of how high your willing to take it before turning it off.