Do you use a wedge cushion? The type I have has a cut-out at the back which may help with tailbone issues. IIRC, it came from Staples (in the UK but you should be able to find one at any major office supply shop or order from Amazon - like this one
https://www.amazon.co.uk/Medipaq-MEM...osture+cushion). I don't think mine is a memory foam type though - it's probably too old. It also helps if you have the type of chair that doesn't have seat tilt as it mimics the tilt effect.