Tablet space for apps from its own internal USB storage
Firstly, I do not think that you can install more RAM on most Android devices as they have fixed amounts of RAM. I would suggest that you minimize background processes in order to free up more RAM.

Secondly, I also do not think that you can install more Internal Storage to an Android device, again because the amount is fixed. The only way is to add an SD Card/MicroSD Card which you do not want.

If you install apps to USB Storage, don't you have to constantly keep it plugged in in order for them to run?