Patched that exact mistake last year with a cheap welder from harbor freight and some thick sheet metal. Cut out the rusted spots, welded patches over them, and coated the whole firebox with high heat paint. Been holding up fine through two seasons now.
Yeah I read something similar actually, some guy on a homesteading forum said he used truck bed liner on his firebox instead of paint. Said it held up even better than the high heat stuff. I guess it creates this thick rubbery layer that doesn't crack when the metal expands and contracts. Might be worth looking into if yours ever needs a touch up. Though I'm sure the paint is fine too, two seasons is pretty solid for a Harbor Freight weld job honestly.
And that right there is basically every DIY project in a nutshell, you fix one thing and realize you missed three other problems. It's like life just throws these little pop quizzes at you to keep you humble lol.