Acidic strength of para substituted benzoic acids
You are somewhat on the right track. However, consider that acidity depends on the relative stabilization of the acid vs. the conjugate base, since the expression for $\mathrm{p}K_\mathrm{a}$ is as follows: $$\mathrm{p}K_\mathrm{a} = -\log\left(\frac{[\ce{A-}][\ce{H+}]}{[\ce{HA}]}\right)$$

Consider this figure for a visualization:



Electron-withdrawing groups like p-$\ce{NO2}$ (nitro group in the para position) increase acidity by making the carboxylate ion stable relative to the protonated form. However, electron-donating groups like p-methoxy decrease acidity relative to standard benzoic acid by destabilizing the (already negatively charged) carboxylate anion!

TL; DR: An electron-withdrawing group like p-$\ce{NO2}$ will better stabilize the negatively charged $\ce{COO-}$ group than an electron-donating group like p-$\ce{OCH3}$.

I found a neat picture from UC Davis that summarizes what I have said: