I prefer middle splits with knees forward, this position is more "comfortable" to me. I tried knees up and I was able to get lower initially, but not to hold the position long because my feet, especially my heels, hurt! What helped me to get comfy with knees forward was to work on hip rotation and strengthen the hip rotator muscles (gluteus minor maybe?). For that, you can for example stay in your half middle split and rotate the straight leg back and forth in the hip socket. You can also do it from all fours, lifting your leg straight in the air behind you. Or from a straddle, laying on the floor and legs against a wall.
Also, when you are close enough to the floor (a few inches, maybe 3-ish), you may want to give a go to "oversplits", placing one foot on a block and the other on the floor. Always do this when you are super warm and be very careful! This is pretty advanced.
Middles are hard and take time, I think that besides getting muscle elongation, your body needs to get used to the awkward position and learn to relax and not to freak out... which again takes time.
Last but not least, consistency is key. Even if some days you cannot go very deep, you are training you body to get familiar with the process and position. I personally measure my flexibility improvement not only on how flat or squared I am, but also how I feel. Am I relaxed? Can I hold the stretch longer? Everything hurts and I'm dying? LoL I think stretching deeply 3 days a week is a good dose. 4 at most.
So keep it consistent and slow :-)