I'm no fan of hydrofoils as they just add drag and sometimes odd handling... but have to admit sometimes when nothing else works in changing your setup, engine height or trim then they could just be an answer.
Re the outboard depth well yes to many here including myself the AV plate (cavitation) level or 1" below hull bottom at transom is the starting point and often ends up being the right level. But if you are already suffering ventilating when a bit deep raising could cause more of the issue... but air floors do not behave by the rule book so I'd try it raised with a packer to see what difference it makes. I'd be trying at least an inch raise or a bit more if the outboard clamps allow.
Image attached of my 360 and Mariner 15 2-stroke below... just for interest. I didn't use any packer with that setup because despite being a bit low (perhaps 2" under the hull) it had good speed and no splash.
Video of it below too. I had just one passenger and when I straighten up around 36sec I take it to maximum speed and you'll hear there is no slip. Speed towards the end would have been about 23mph (20kts).
The slip was rarely noticed with a minimal load... nearly always when 4-up which makes sense as the prop is looking for more grip to push the load.
So for you I'd try... make 100% sure it is up to pressure once it's been in the water a few minutes as SixtyNorth advises and ensure you are happy the gauge you are using is accurate... ensure the outboard trim is set midpoint... ensure your passenger load brings the boat as level as possible... try a packer to lift the outboard 1-1.5"... try with just one passenger and see if the problem is the same... consider pitching the prop down to a 9" which is what my Mariner was fitted with... and then finally possibly a hydrofoil.