It usually hangs on 6% for a short time which is normal, but this stays at 43% and that's as far as it goes. Just prior to that I checked the validity of the ORBX Files.

Almost certainly there will be an add on scenery/airport that has a file which can not be read.  A slow process to fix but try.

remove all add ons.  restart.

if it does not start look elswhere for a fix.

If it starts

add half the scenery,  if it starts, that part is fine so remove the scenery to another folder temporarily.

restart to test.

add the half you did not try

attempt restart. if starts ok look elswhere for a fix,

if it does not start remove half of this scenery.

repeat these steps.

eventually you will find an addon or add ons which have a problem

remove these.  Add all the rest and restart the sim.  If it loads you are fine if not look elswhere.,