If that is as flat as it looks in your last picture, then it does need more dihedral. You should be able to bend this in easily.
What are you using to control pitch trim on this? I dont see elevons nor any canard......are you just going to try to trim and glide a straight up delta without any of that? That is darn nigh difficult at best, and even if you think you have it trimmed it may not act it in midair, especially if it is moving rapidly nose down at ejection. Or even if the wind affects it badly if you had it trimmed to do well with calm.
You might be able to very slightly bend the trailing edges up a very limited amount, just barely enough to see, and this may get you a repeatable glide. Of course, this can affect boost, since you would need a more forward CG to offset this.
Either way, keep posting your progress on this. Always gotta try and learn something along the way....