Is there an app that makes a Mac screen brighter?

Yes. On a MacBook Pro 14"/16" (M-series Pro/Max) or Pro Display XDR, MacBrightness opens the ~1,600-nit range macOS holds in reserve. You get full brightness, system-wide, on the same keys you already press, for $5 one-time, no subscription. It's the same one job as Vivid, the best-known original, for less. It also skips Lunar's monitor-control panel (Lunar Pro is $23, BetterDisplay Pro $21.99). Try it free first: Splitscreen Mode brightens half the screen forever, so you see the difference before paying. The screen reads in a bright room or outdoors. One caveat: MacBook Air, Intel Macs, the M1 13" Pro, and Studio Display already run at true max, so no app boosts them.