Playing Gears 5 without Gears of War 4 might be weird. All new characters were introduced there so you would definitely miss out on story. Just like someone else said, start with Gears Ultimate. 13 years old but plays and feels timeless.
The Outer Worlds is indeed quite short for an RPG. Think Mass Effect 1 in terms of length. I got through the story, completing most side quests in 18 hours (according to in game tracker). Do me the favour and start the game on hard though. It's too easy on normal. The first two hours might be tough but it's getting pretty easy later on.