Brewzilla gen 4 users, I need your guidance

I used to brew in a digiboil 35L with BIAB in it. The beer was good, efficiency a bit on the lower side (65-68%). Without recirculation, it was very hard to do a step mash so I decided to buy myself a brewzilla gen 4 with the RAPT bluetooth thermometer and the heat exchanger dish/HED.

So far it looks like my worse investment in brewing gear ever.

I can't make a step mash working as the pump will create a vacuum under the grain bed.

Here is what I try so far:

- I rewired the pump

- I removed the top filter in the grain pipe (I think this actually helped a bit, not sure)

- brewed with and without HED

- Putting a BIAB in the grain pipe (no more clogged pump or bits of grains carried into the fermenter)

- crushing coarser + 5% of the grain bill as rice hulls

All of these have failed at preventing the formation of that vacuum, impairing wort recirculation and therefor not being able to heat up the mash.

Worth mentioning: I brew 13L batches (3,5 gal), with grain bills between 2 - 4 kg (4,4-8,8 lbs).

I start by mixing my grains with the water, and let them sit for 10 min and then open slowly the pump valve (pump at 100%, I just use the valve to moderate the flow). When it looks like it is compacted and the mash needs to heat up, I open the valve more. I see then the mash water lever rising, due to the vacuum at the bottom.

I knew there would be some room to optimize but nether thought that much trouble. Brewhouse efficiency is garbage (60% no later than today) and if I can't figure this out in a reasonable timeline, I will just sell it and go back to my BIAB: Beers used to be fantastic.

Looking forward your input on that matter. Thanks.