You can get an installed system into a USB stick 'without too much effort' like this, as a start. Later on you can use more complicated methods.

1. Download a compressed image file of an installed mini system and check that the download was good.

2. Install mkusb into a simple live Ubuntu based system (with your limited RAM).

3. Use mkusb to install from the compressed image file to the USB stick.

4. Boot into the USB stick. Now you have a system with simple menus. You can select which desktop environment you want via a menu and reboot, for example Ubuntu MATE.

5. Expand or 'grow' your partition and file system with gparted, so that you use the whole USB stick.

6. Install your favourite program packages and tweak the system to what you want.

7. Make a backup of your system, and get a routine to backup the whole system, part of it or at least your personal files with regular intervals.

See these links and links from them with more details,

Installation/UEFI-and-BIOS

https://help.ubuntu.com/community/mkusb