I think something a little more upright is required
Road tyres on your mountain bike?
Granted I’m not 16 stone but I have wavered between 13 and 14 for some years. I’m 6 foot too. I bought a bottom of the range Trek road bike for £250 over 11 years ago. I’ve had to replace the wheels after a wrong turn took me onto a rather bumpy lane, but otherwise it is still going strong.
My old man has one of those hybrid/commuter style bikes and loves it. More upright, more comfort and it serves him well enough up and down the Peak District hills but is good for the “family” trails too.
If you know nothing about what you’re after my advice would be find a better bike shop and look at last season’s models if you want to save a few quid.