If you are seeing good groups with the slugs you have tried, and the only problem is that they hit low of your point of aim, then you can solve the problem with a lower front sight or a higher back sight. You could change your sight picture I suppose, but that may not be comfortable for you. Many folks get the bead settled in the rear sight, and place the bead direclty under their desired point of impact (6 O'clock hold). If yousight that way, try covering the spot you want to hit with the front bead and it will raise your POI.

I have a 500 that has the front bead sight welded or soldered on. If your 535 has that kind of front sight, you really can't do much with it. A front post style sight can be filed down to raise the point of impact. There are probably some different rear sights you can install. The rear on my 500 in mounted in a dovetail slot, you drift it out to change it. Mossberg makes a peepsight, "ghost ring" or something like that. Might be worth a try. I have my rear sightas high as it will go, and luckily it hits right on at 75 yds, which is about my limit with open sights.

If nothing else works, try a slug that is moving SLOWER than those you've tried. Seems funny, but they will hit higher than faster slugs (within 100 yds or so), as they stay in the barrel longer and exit later during recoil.