This already exists in-game. Under your character config go to the Hotbar settings > cross bar settings and there is an option for "Expanded hold controls". You can set up an extra set of 8 actions that show up by holding L2 + R2 and you can even have it so you access 2 different sets depending on if you hold L2 or R2 first. Keep in mind that as the name implies this function only works if you have your crossbar set to Hold or Mixed, Toggle won't work.