unless you screen is OLED, dark-mode has no or zero impact on battery usage, maybe even non-significant impact on OLEDs

HP Envy does have OLED screen options. I'd assume it's what they have, if they thought dark mode was relevant.

Don't most LCD screens have localised dimming of the backlight these days?

Not laptops. Local dimming zones look awful when you have a white cursor moving around, so it's mostly still just a TV-feature

Looking awful has not prevented local dimming from becoming quite common on laptops. Apple has been doing an okay job of it in the MacBook Pro for several years. Lots of Windows laptops have been very hit-or-miss about it, but at least with those you often have an OLED option. I've seen multiple Windows laptops from more than one OEM where opening a terminal window with light text on a dark background means you can easily spot a single line of text getting much dimmer toward the center of the dark window, and lighter near the perimeter where it's close to other light content. And that's for static content; as you mentioned motion can bring more problems as the backlight lags behind the LCD.

I'm using CachyOS on my framework and after switching from windows gained about 30%-50% battery life depending on exactly what I'm doing.

Wow. For as long as I can remember it was usually the opposite for me. Even after configuring TLP and looking for things to tune manually with powertop, I usually didn't get battery usage quite as low as I wanted. I thought it was still typical for Windows to be better.

Are you running a DE or just a lightweight WM?

I’m running a Hyprland-based setup, so more WM/compositor than full DE, with a small bit of tuning in Arch’s power settings.