If you want to increase the overall loudness then you should use dynamic compression. You can either use the audacity’s built-in compressor or download chris’ compressor: http://pdf23ds.net/software/dynamic-compressor/
If you search this forum for dynamic compression or compressor you should find a lot of threads about the subject.
You might also give a try at steve’s recent equal-loudness plugin: https://forum.audacityteam.org/t/equal-loudness-normalizing/19112/1