Here are links to two plugins for Audacity which reduce mains hum …
Hum removal plug-in Steve’s “Hum remover”
Multiple notches with Nyquist prompt? - #10 by Trebor Trebor’s “Mains dehummer”.
Try Steve’s first, if there is still too much hum remaining try mine instead.
It’s a trade-off : the more hum removed the more it sounds like you’re listening via a tube.