Unless your angle grinder is mounted on a stand, with an attached work table, I'd have thought keeping the blade at 90° to the wood would be rather difficult. Also, with a 90mm thick piece of wood, you'd need to have a very large angle grinder in order to get the depth of cut you'd need. A 230mm/9" grinder - the largest hand held size I can find available - has a maximum depth of cut of 65-70mm (or so the web says) in one go.

I'd be thinking of different tools for the job. I've got an electric plane and belt sander, neither are hard to use (though you need to be careful to not get too enthusiastic with them!) and I'd be tempted to do most of the work with the plane and then swap to the belt sander.