Hello and welcome to the forums - great to have you with us
We've had a few members joining recently who have made the jump from smartphone to DSLR - it's nice to see that happening.
The K-S2 was an excellent choice. As for macro lenses...
I would recommend you consider the
Pentax-D FA 100mm f/2.8 Macro. It is one of the most popular choices, and for good reason. It's not a cheap lens, but the price is actually very reasonable considering its performance - and you can find good used examples at reduced cost. Earlier versions are less expensive still (though none of them is cheap).
A lower-cost alternative to get you started would be a macro conversion lens such as the Raynox DCR-250, which clips onto the front of a non-macro lens. I've seen some excellent macro photos taken with this kind of setup. Cheaper still, you could try out some close-up "filters" (not strictly filters, but close-up lenses that screw into the filter thread of your lens). These are inferior to a proper macro lens, but can still provide very satisfying results