Grinders don’t work particularly well to remove aluminum alloys. Aluminum simply loads up the the wheel. I’ve had to remove stuck seat posts by cutting them out.

Cut the stem / seatpost off leaving a fair amount of space above the frame. Then cut a slot along the length of the remaining cyclinder still inside the frame. You don’t want to try to cut all the way through; you’ll probably cut into the frame if you do. You will create a slot with a slight dome in the middle, thinner at the ends. Take a cold chisel along the length of the slot to create a crack in the remaining web. Cut a second slot nearby using the same technique. Somewhere around 1 cm is appropriate. Now you can use some sort of punch (including the chisel you have in your hand) to pry the section back. Now you can take a pair of channel locks or visegrips to squeeze the remaining post away from the frame. You may have to repeat several times.

I’ve used hacksaw blades which was an all day job, but it became manageable when I used a sawzall type electric hacksaw. Less than an hour to remove the seat post the last time, probably 30 years ago.