From the pics you have posted your side hair looks fine enough to blend with a topper cut shorter rather than leaving it longer, imo. Can't really see the back as good in the pics. I wanted to keep my last piece a bit longer but at time my sides were cut short. Although not thinning on sides my stylist said harder to get blend right with shorter sides/back to keep topper longer, so from your pics I would say short. I see you posted this a few days ago and already had cut in so curious on which way you went.