Suggesting a file manager is somewhat difficult to do, given the number of applications available.
Taking into account that you have asked about Fluxbox, knowing the characteristics of it, my first suggestion would be that you use the Midnight Commander, which is in the repositories.
But reading what you would like the file manager to have, my suggestion would be to use Sunflower.
To install Sunflower:
Open up your terminal and run the command below in order to download the .deb package :
wget -c https://sunflower-fm.org/pub/sunflower-0.4.62-3.all.deb -O sunflower.deb
sudo dpkg -i sunflower.deb
you may want to check if the newly installed package needs any dependency, run the command below:
sudo apt-get update
sudo apt-get install -f
If you don't like it, uninstall it with the commands:
sudo apt-get remove sunflower
I estimate that you will receive more suggestions, due to the number of file managers that meet your expectations.
If you keep Sunflower, to keep you updated from time to time you should visit this site: https://sunflower-fm.org/pub/