The back of my aquarium has slopes up to 8inches of Aquasoil.. I'm preparing to start DSM of HC. Should I be concerned about anaerobic problems?
Yes.
You can do some things to help minimize the risks (plant roots and Malaysian Trumpet Snails can help), but keep that risk in mind. Any time you do any work in the tank I'd recommend a very large water change afterwards just in case you might hit a bubble.
I had a massive fish die-off some years back after pushing my 90gal from one room into another that I'm pretty sure was from disturbing some H2O2 pockets (my best guess, at any rate).
Most people say 2-4" because that's really all that's needed by most of the plants we keep. Only gignormous plants (like swords or crypts) will have much deeper root systems.