Is it safe to increase MacBook brightness past the maximum?

Yes, with a caveat. It's safe because it doesn't touch your hardware. It just opens the brighter range Apple already sustains for HDR video. That's the brightness your panel is built to hold: about 1,000 nits indefinitely, up toward 1,600 peak. The slider stays under macOS control, so there are no low-level hacks and no noticeable CPU overhead. Your Mac wasn't holding back because the panel is fragile; it held back to save power. That's the real tradeoff: a brighter screen pulls more battery and runs the panel warmer. The mini-LED is fine at these levels long-term. Full brightness all day mainly costs battery, so dim it indoors and brighten it outside.