Well, loading means making the tcz available to run. Even when you are not running, it will be loaded if it is on onboot.lst.
Looks like you don't need some extensions. Using the Apps browser, when you download, click on "OnBoot" and change it into "Download + Load". This makes the extension to stay on HDD but it doesn't load them. OnBoot means they will be loaded at the boot of TCL thus consuming more time and more memory.
If you want a shortcut to load that extension with just a click, you can change that "OnBoot" to "OnDemand". That makes that particular extension available in the OnDemand menu.
... such as screen brightness ...
For screen brightness, you can install lxrandr packages - lxrandr-locale.tcz(for different languages), lxrandr.tcz and lxrandr-doc.tcz(only if necessary).
This is some complicated. But if you don't want to install them, test this and if it works, you can add this as it is into "bootlocal.sh" :
MONITOR=$(xrandr | grep " connected" | cut -f1 -d " ") # Get the monitor name
xrandr --output "$MONITOR" --brightness [brightness-level]
The brightness level should be set between 0.5 to 1 for better visibility. You can edit brightness for next boot by editing bootlocal.sh without any GUI tools.