Well, levels and numbers can really be a rabbit hole, it's a very interesting topic and an important part for sure, but it can be a very tricky one as well. But you can always push the door of this rabbit hole, should you wish to.
1 entrance below
But rule number one always stays rule number one : " if it's sounds good, then it sounds good ! "
In the end of the day, no ones care "how" you produced and mixed your piece or art if they like it.
That say ( and answer below ), yes, those levels you mentioned are too low if you aim at competitive mixes.
Mixing at low levels can be a good habit IMO but this has to be done per channel and instrument or audio output ( so your channel source ). You can adjust the output of a VST instrument or the output of an audio clip, you can also place a Utility or Gain device to adjust this output.
So that's a first level of control that can be handy if you process your instrument and audio with plugins that emulates vintage gears and usually work best with an input level around -18dB
After your instrument/audio clip + Fx processing on each track, you can then adjust each track output to "balance" your all Mix.
The sum of all tracks goes to your 2 bus ( Master Channel ) and when it comes to Mixing the final version you want to render, always keep your Master Channel to 0dB and use a Limiter to increase your level if you have for habit to Mix at low levels.
I would say that your LUFS values and true peaks values mentioned in your previous post are way to low, IMHO you should aim between 14 and 12, even 9 LUFS value with your meter peaking around -2 / -3dB if you're aiming at a competitive final Mix.
LUFS and True Peaks can also vary depending if you plan to upload your Mix on streaming platforms since they will process your final track according to their standard.
Some plugins will help to set "Targets" for your levels and can help you to reach the final track loudness you're aiming for.
That's when you want to puzzle your head and eventually end up with your brain smoking around numbers and try to mimic a real world console + analog gears mixing scenario.
So like Phil mentioned, important bit is not to go in the red, and you can always use a gain plugin on your Master channel before hitting your limiter if you find your Mix level to be really to low, but yes, I would say that those LUFS and True Peaks levels you mentioned are to low.
Some interesting SA tutorials you may want to watch :
Loudness and Metering with Kirk Degiorgio
Understanding Mixing Fundamentals with Phil Johnston
Creating Wide Mixes with Protoculture
Top 5 Plugins for Loud Mixes
MasterCheck with Dom Kane
Mastering The Mix - EXPOSE with Rory Webb
ADPTR Streamliner and Mastering for Streaming Platforms with Protoculture