Question is this psu fine if i swapped out the 3080 for a 6900xt ?

That should be up to him, if he is sure he wil get more fps to fully utilize his monitors refresh rate than that's up to him. He can still sell the 3080 .
Of course it is up to him, But ya know we like to advise people of the point of dimensioning returns. According to benchmark with similar setup to his current 5800x3d 3080 etc. he should already be getting close to if not 240hz depending on in game settings. Then again the 5800x3d is good but at 1080p I don't care what it is paired with even a 3090TI getting past 240 is going to be rough. Cpu bound at that point.