He's not left footed. But he's played left back many many more times than right back. He's experienced at left back. What I would ask is how much is Spence being 'wrong footed', and how much is the lack of established left winger? Udogie hasn't pulled up tress either with his left peg.
I'd say getting the team settled, with players linking up and working well together, is more vital than a players prefered foot. Spence defensively is amazing. Sensational. Underated. He earnt a call up to England playing left back for us. When 1-1 against the best attackers in the game, he's hyper focused and alert. Strong, quick. And going forward there are signs....its not clicking yet. The potential advantage is Spence being able to cut in, to curl crosses towards the far post from deep, and he is perfectly capable of pulling back the ball accross goal with his left peg. I would say the limitations are not so much about his footedness, and more a need to settle the left side. Spence has probably looked the most up for it of all the incumbants along that side of the pitch. Whether with Muani or Odobert, we need Gray (or the CM of the day) and forward to help create triangles and help open up that side of the pitch.
Signing someone likely won't change that. Only a handful of players beat defenders without movement, team coordination, stretching defenders out of position etc.
So you sign a new LB. Do they need to be better than Spence and Udogie? Who is that? And then you have to sell one of Spece and Udogie when fit as they won't get enough game time.