QMK firmware has a handy feature for this called Tapping Force Hold[1]. If you hold the key normally, it modifies the layout, but if you tap it once (or more) and then hold, you'll get the normal repeating behavior.

Example A:

Hold Backspace => Activates layer overlay

Example B:

Tap Backspace => Input backspace

Hold Backspace => Input held backspace => OS handles repeating as configured

[1]: https://docs.qmk.fm/#/tap_hold?id=tapping-force-hold