I love theme 2, but it's slight, not even quite a thing that feels like a real theme. It's almost more a rhetorical conceit, that a variation on the opening riff is given alternating dynamic markings in two-bar patterns.
But it works wonderfully. Matiegka had an understanding that some of the other guitarist composers didn't always display, which is that for a sonata form to work thematic/structural differentiation is critical. We need to have an understanding that we've arrived at the new idea and while Matiegka's ultra stripped-down texture might seem like too simple a way of announcing the arrival of theme 2, the theme is simple and charming. It works. Or maybe it doesn't, you're free to disagree, obviously.
As is common for guitar sonatas of the time, Matiegka ends with a flourishing coda that's pretty much a I-V alternation of the kind you're going to find in pretty much every sonata form from guitarist composers in this period. But it works nicely. In fact, at least for Wenatchee's time, this sonata exposition was so fun it was disappointing Leisner didn't take the structural repeat specified in the score on his recording. Compared to the other guitar sonatas, even those of Sor, Matiegka's Grand Sonata I has an exposition that I found wanting to hear a second time right after I'd heard it. That is a practical measure of how well the exposition works. Matiegka's not a genius like Haydn, but his best moments are still worth our time.
What sets Matiegka apart from other guitarist composers from the early 19th century is that his development sections are actual development sections.