the easiest cross platform way is to use qmk and flash some firmware on the keyboard itself to map that. tons of software tools for this now

That's what i did. It's worked super well. Finding a keyboard i liked that worked with that was the hardest part. As it is my keyboard's brand (Keychron) had support in QMK, but the model did not. I had to hack a bit to get there, but in the end it worked.

Sad thing is that was two years ago and i've forgotten how to flash it lol.

I can't wait until something like Rust's QMK (i think RMK?) starts to work on more boards. My board having complex modes but the ease of it being written in Rust (because i love Rust) would be a dream for me.

If i could i'd write those type of Rust-QMK for both my Keyboard and Mouse. It really feels great

While that's true, QMK isn't context aware. You can't have application or OS specific mappings for instance.

Edit: typo fix

This looks like it might work on my ordinary Dell Latitude laptop, which certainly doesn't support QMK for its internal keyboard as much as I wish it did!

May I suggest the absolutely awesome Extend Layer [0] instead?

Arrow keys, advanced cursor movement and modifiers chording, and more, without leaving the home row!

[0] https://dreymar.colemak.org/layers-extend.html

it would help to know what your tried. kanata?

Yes it does but a lot more. I've personally switched to using only Keymapper even though my keyboard runs on QMK. There's a lot of features but the killer one for me is the context awareness. You can have different mappings based on the application, device or OS.