Hi. I'm a refractory designer. I have over 30 years of experience in creating linings for industrial furnaces with bricks and monolithics, including what we would call 'Precast shapes' of which this would be included.

Those are definitely air pockets stemming from the pour that were not vibrated out. It could be your aggregate size is making it difficult. I don't know the product you are using, but often there are 'fine' versions that feature a smaller grain size just for better consistency when pouring. I would never recommend gunning something like this.

As far as vibration goes, they sell industrial vibrators with long rubber mixing tips. I might have video from a few of my precast shapes showing a large precast shape getting vibrated. Everything about it just screams dildo and you feel silly, but it is what it is, and it does work.

You'll know if you add too much water. It will be brittle and fail if you can even get the mold released. More water is never a solution. Better PH is superior to more water.

Vibration can be achieved by building an outer box for your foam insert. Wood can work for something this size probably, but metal would be best. You can then use any vibratory method that works. Pour direction matters too, but it opens ugly doors like lifting and drying.