If you can blow through a 5 pin Schlage, then get your hands on a 6 pin commercial Schlage mortise cylinder and a 6 pin Arrow KIK or mortise cylinder, Sargent is another well made lock that should keep you busy. I find that extra pin stack adds to the challenge. The other advice already posted about American Lock padlocks is good as well. Sometimes on ebay you can get a 7 pin Yale cylinder in mortise or LFIC. They are sometimes pulled from existing systems and may be "easier" because of master wafers in the lock.
a Sargent 6 pin I just found for you as an example:
www.ebay.com/itm/121656143849 Congrats on reaching a stepping stone and looking for more of a challenge,
Squelchtone