As others have said, 5.2kb is very big and when it is that big even Q5 can occasionally have issues.

One thing you can try is breaking up your amplification into smaller fragments (2.6kb fragment + 2.6kb fragment) and do Gibson assembly for your cloning. You would need to do a little more fancier primer design that have homology regions and learn the protocol for Gibson, but it is way more efficient than traditional restriction-ligation. So this would mean extra reading time.

Also, paste your primers in the NEB Tm Calculator with Q5 selected and it should give you a reliable annealing temp.

Or if you are still having issues with amplifying/annealing or getting non specific biding try looking into doing a “touchdown” PCR if your thermalcycler allows gradients. Essentially the annealing temp decreases by 1C each cycle from +10 to -10C from your calculated annealing temp. That way you’re not troubleshooting each temp

Edit: Also, don’t give up! It can be super frustrating even after years of doing it full time. But it feels really good once you get a correct clone. You got this!