its worth cleaning the whole tape path with alcohol - pinch roller et al
if youve tried all the non-invasive options, its possible your head is out of alignment. the procedure is straightforward and no special tools are required. i had to do it to one of mine:
zero the tape counter and attempt a load. make a note of the counter number when the load fails.
make a small turn on the tape head adjustment screw and re-attempt the load procedure.
if the load fails sooner/earlier then the adjustment was in the wrong direction. turn the screw back and also a bit further.
re-attempt the load and note the counter number where the load fails - it should have got further. if it has not then alignment is not the issue, turn the screw back to its initial position.
if it has then turn the screw further in the same direction, note the counter and repeat the process until the alignment is reliable/satisfactory and the game loads in.
if i recall correctly, zaxxon loads at between 110 and 120?
always use start & option on the 800XL unless the game requires basic. when the XLs came out, so many games were already in circulation for the 400/800. the game instructions had no reference to option being pressed because 400/800 didnt need it - users just physically removed the basic cart instead. this befuddled me for a short while as a kid - until i read the XL computer instructions! (hehe)
best of luck