Never having used the audio tools much in shotcut so sorry there.
If you are dealing with dialogue, I’ve found you will need a lot of compression to get to -14 LUFs. I sometimes have two compressors and can barely get to -20 LUFs
Playing it alongside other videos I’m happy enough with those levels.
Another thing might be to look at your audio with a spectrograph / loudness meter you can see over time where the levels are peaking. This is from Ardour 6.9 when you export your audio. You can see where my audio levels are peaking 2/3rds away along the track. So if I gain stage that area with with the sliders and bringing the levels down with that peaky particular part of the video I can raise the rest of the audio either with compression or faders/gain.
But to get to your main question -14 LUFs I can’t see doing that with audio dialogue and not applying compression. If dealing with premastered music that will be much easier.
Sorry I am not sure how to get that level of granularity from Shotcut. The compressor is there but I’m more comfortable with the ones on a dedicated DAW. I hope this helps.