Since version 2.4.0 (May 2020), Audacity offers a Loudness normalisatin tool!

See release note.

Find the tool in the Effect menu > Loudness Normalizazion...

The legacy Normalize... tool is normalising based on sample peaks.
► The Loudness normalisation tool is recomended now.

It event takes care of the Stereo VS Mono challenge.
Just keep Treat mono as dual-mono ticked and normalise at the recommended target.