Remember that SU is a SkyTeam member. Have you checked their fares? They interline with DL and probably have codeshares with AF/KLM, so you might even be able to get a SU ticket with all segments operated by DL/AF/KLM. Of course, there can be disadvantages of doing it this way, but it might be a means of getting a cheaper fare on SkyTeam.

If you go with two separate tickets, you could build in an overnight at AMS and still save lots of money. However, note than many KLM tickets (and those for other European carriers, including cheaper business class fares) must be flown exactly as ticketed or all value is lost. No changes whatsoever except for involuntary changes if there's a schedule change or IROPs.