I've never used an SY85, but have used a lot of Yamaha synths and have had a CS2X for a number of years... I've noticed the AWM2 TG in them all can sound quite a bit different from each other since you have to account for the entire synth engine, what comes after the TG, to properly judge each synth's characteristic sound. Seems like you want to ignore all the important parts of the SY85, I don't think I could do that, the workstation features would be a huge plus if you don't have something to sequence the CS2X. The arp on the CS2X is not programmable, you are stuck with the same arps and they are not too bad, but become useless and boring over time. The other thing is, even if you get a sequencer, like a QY70 which does go nice with the CS2x on its tray, but anyhow you are limited to using the CS2X's performance sounds to just one track/channel. It is at heart mainly just a performance synthesizer. The rest of the tracks/channels have to be relegated to having to use the XG sounds, which arn't nearly as editable. There may be an exception there for drum sets on track 10, but unsure of that or if it matters much, been a while since I've sequenced my CS2X. Not sure about SY85 either, but if you are able to have AWM2 performance sounds across more than one track/channel it would be a no brainer for me, I'd keep the SY85 and forget about the CS2X.
I keep my CS2X mostly for one particularly great sounding, grungy-distorted guitar, which from the YT demos I've heard of the SY85, it seems like it has a lot of great guitarish sounds too, but I didn't hear the one particular patch/voice that I love on the CS2X, if it didn't have it or be able to recreate it, then I would probably wind up keeping both synths, just for one dumb patch
Also, does the SY85 have a dual HPF + LPF filter with distance? HPF with LPF is nice on the CS2X, basically can set their cutoff distance on it quickly since they have separate knobs, or maybe its just two chained filters I'm not sure, but having them paired like they are on the CS2X I think makes for some unique sounding sweeps.
Anyhow, I'm unsure of the prices they go for now days, but have you looked in to the older Motifs, or an RS7000 at all?