If you want to try it yourself and aren’t in a rush then I managed it recently on a bike where a good 20cm of post was stuck in the frame and no amount of brute force or release oils were working. My techique was:
Buy new hacksaw blades from a good manufacturer (Bahco) with the lowest tpi I could get (18). The idea being that normally you use high tpi for steel and low for aluminium so I thought that lower made it less likely to cut into the steel. This technique isn’t going to work if your seatpost is in longer than the length of a hacksaw blade so check first.
Cut off the top of the seat post about 4cm above the frame. Enough to grip with huge pipe wrench but as little as possible so I could get as much hacksaw blade in as possible
Find a piece of dowel that would just fit into the seatpost with a few mm to spare
Cut a groove into the dowel with a multi tool, the length of the blade and deep enough to only leave enough blade exposed to cut through the thickness of the post plus a teeny bit
Jam the blade into the dowel but with the teeth set to cut on the pull stroke so you are starting cutting at the end of the post rather then the top. Just to try and reduce the damage to the frame
Then just stick the down in the seatpost and gently try and saw it out. I did two cuts about 90deg apart with the idea being that I should just be able to peel out that piece and the rest would come out with a pipe wrench
In the end the piece didn’t come out but I was able to bend it in enough that the pipe wrench could squeeze the rest and the twisting it from side to side while spraying more release oil in freed it a little more each time. Eventually it came out with only a slight score where the blade had been. Didn’t look more that a scratch really, I was very pleased
The main things were brand new blades that cut with almost no pressure, the stuff the blade cut out looked like wire wool, it was beatiful
The other thing was not trying to do it all in one day. Whenever I got tired and wanted to rush it I just put it away for the day and came back. Took about a week of sawing carefully to get through.