Debian 11 – Low Entropy Issue Even with Haveged and RNG-Tools

I have a problem with an old laptop.

I have installed a debian 11 operating system on it a year ago, and despite using it almost daily to perform network administration, I recently found its available entropy value is low, and don't seem to vary over time, or over activity.

# cat /proc/sys/kernel/random/entropy_avail
256

I have haveged and rng-tools installed, with HRNGDEVICE=/dev/urandom as configuration for /etc/default/rng-tools-debian.

Both rng-tools-debian and haveged services are running without errors.

My question is, how can I increase the entropy of my system ?

Thanks

System information:

• Laptop ref = Aspire V3-771G
• CPU = Intel Core i3-2328M
• Quantity of RAM = 6 Go
• Desktop = KDE Plasma
• No internet access, I manually enable internet access to apply security updates (1 time per week)
• I have installed haveged a while ago, rng-tools this morning, the entropy value didn't vary after the installation of rng-tools

EDIT:
I have a virtual machine (debian 11) running on a Windows 10 host (virtual box), with no desktop and no haveged, and I can see its entropy value going up quickly, reaching 1600+ in less than 1 minute.