Isn’t this less secure compared to KessPass itself? No option present to gather additional entropy using different methods such as keyboard/mouse/hardware.
This is not required. With a proper design, not every application needs to re-invent the wheel and gather its own entropy. The kernel should provide sufficient entropy and applications only need to properly make use of the kernel API.
Related:
As for implementation details, you need to check KeePassXC directly.
Unspecific to Whonix.
Can be resolved as per: