I couldn't find one that was six inch, but I did find a hole saw that was 178mm
https://www.bunnings.com.au/kango-51...51178_p0354588
...and some of them are diamond / carbide tipped, so that may be one approach. If there's a 178mm in existance, there may be a 150mm...although you'll no doubt need a fairly powerful drill behind it, and the patience to work through it carefully.
Just a thought.