ok, 86cm is about 34inches...that IS deep.
You can certainly build a slope, or use plexiglass to form a shelf or terrace to raise the plants up.
But even then, you're gonna have issues. those hex/octagonal tanks are not very well suited to plant keeping, due to the increased depth and the lack of width your lighting options are few.
I suspect you'd have to build up the bottom a little, AND switch lighting to something more intense. 8watts is nothing. Vallisneria like at least 1.5watts per gallon in my experience. If this is a 30gallon octagonal tank, you're gonna need at least 45watts of lighting, preferably more due to the depth.
Please share a few more specs on your tank...mainly the diameter and total capacity.
I'm suspecting you'd want a 55watt power compact fixture, but it may not fit well over your tank...and still may be insufficient for anything but java fern, crypts and anubias bartari v. nana
Former advisor and planted tank geek...life's moved on though.