The slide release should snap back down to the frame after you lift it up. Check to make sure the spring is underneath the locking block pin. It looked like it was, but it doesn’t seem like the spring has much tension, so be sure it didn’t slip behind the pin.
You gonna put a red dot on there? I upgraded my Glock 19 Gen 5 using Zaffiri. I used an Apex trigger and installed a holosun 507 C Red Dot. My Glock 26 is a Gen 5.