Dear Miner:
Regarding trade-in value of a new-ish Delos, I honestly don't know, as each distributor sets their own pricing for retail, trade-ins, upgrades, rebuilds etc. The dealer that you bought the Delos from is probably the best person to ask.
Regarding the next step up for you, the quick answer is the Kleos at US$2700. But 3 months with a cartridge that you say you are happy with, and then onto the next one - IMO this is too quick.
Also, compared to the Delos, the Kleos is a lower-output cartridge, which will be more demanding on your phono stage, and it puts more mechanical energy into the tonearm and damps the headshell less, which means a bigger challenge for the tonearm. I don't know what turntable, tonearm or phono stage you are using, but unless they are up to the task, it could be that the Kleos leaves you less satisfied. I recall some Helikon users who upgraded to the Titan, and for them and their audio systems it turned out to be a downgrade, while I've previously pointed out in this thread that some users prefer the Delos over the Kleos in their systems.
If I were you, I'd listen to the Delos more, probably borrow a lower-output cartridge to see how your phono preamp gets along with it, and maybe borrow a lower-compliance cartridge to see how your tonearm likes it. If any of the results suggest that there could be problems or marginal results from the Kleos in your system as it is now, I'd then look into alternative phono stages and/or tonearms which could do better. These wouldn't necessarily need to be new - something like a second-hand Graham 2.2 or Triplanar could be pretty nice.
I believe that an upgraded phono stage or tonearm could be a more effective path for you now than the Kleos, as these would give you a better foundation for performance improvements in the future, regardless of whichever phono cartridge you ultimately decided was the one for you.
It could be a result of my engineering background, but I get a lot of pleasure from figuring out how to squeeze out the absolute maximum performance that I can from my gear. Only when I'm satisfied that I've extracted pretty much everything there is to be had from a component will I think about its replacement.
I'd also suggest that you take your time. Unless you have lots of income to dispose on audio, unhurried, carefully considered decisions are likely to get you farther and give you more ultimate satisfaction (and for less outlay).
I'm not saying that you shouldn't consider a cartridge upgrade, but hopefully I've shown that there are other issues and options that you may want to think about first.
kind regards, jonathan carr