@mbugbee I've owned all the stuff you are asking about at the same time, only thing I don't still have is the PA50 because I sold it when I got the PA100R

. But I still have the SLO30, both Syn modules, and 50W powerstation still, had th PA50 at the same time and now have PA100 an have A/B'd all this stuff.
The Syn modules are great, in particular the OG SLO module, but it only gets an you an excellent approximation to that one thing: the OD channel and it does differ vs the amp in a direct comparison. The amp has a little more clarity/openness; the module is little hairier in the top end also. They are close enough that you have to really listen to them when you dial them in to pick apart differences.
The II module is awesome, in its own right but it is more different from the amp's OD mode and easier to distinguish but can still do a lot of what someone would want fomr a Soldano, the crunch mode on the is completely different from the amp's normal channel but is useful...
Honestly, I love the II module now; partially because it is a little different and I have access to a version of the SLO amp so I don't really need something fully redundant. I would not be surprised if the II shares som DNA with the new X88IR which does not sound exactly like an SLO either from what I can tell.
The non-OD channel on the amp cannot be truly recreated on any of the modules, IMHO. It is absolutely special and wildly underrated.
The PS2 is a perfectly good power amp for a Syn module and the SLO's power section is notoriously clean, just like a PS. So I would not buy a power amp just to emulate an SLO type tone if that is the goal. People overstate the transparency of the PS online, IMHO.
That said, the KSR is much more versatile, can take a bunch of different tubes, it's highly adjustable as an amp with wide ranging presence and depth and even has a HiFi mode of its own (for modelers, etc) along with a feedback control that gives the ability to tweak the feel and response beyond what most other amps provide. An upgrade that is worth the scratch if you have it.
TL;DR: Get the amp if you really want an SLO above all else. It is absolutely worth it and it'll never be an approximation of anything and you will never wonder.
If you want lots of options, including something SLO OD channel-like, the Syn1+PS2 could totally get you there. If you go the Syn route the KSR is not a necessity but it would potentially add a new dimension to your rig if you added it or upgraded.