critique my setup -- weak link? best way to improve?

you could also add a sub. I think the big question, are you a "set and forget" type or a fiddler. If you are the former, you can get powered active speakers and sell the Rotel and also upgrade your cartridge. Pro-ject tonearms are good but you can improve the needle. And yes, lastly, a phono stage

Your biggest limiting factor is placing your speakers on a sideboard.  The best/cheapest thing you can do by far is get your speakers on some stands and position them properly.  Upgrading speakers would be an obvious area for significant improvement, but if aesthetics are more important to you than sound quality and the speakers need to stay where they are I wouldn’t spend a lot because the sound will always be compromised.  There are some speakers that are designed to work better close to walls, in cabinets, etc. so I’d look to something like that or at least a speaker that’s sealed or front ported.  Best of luck. 

speakers. Just a reasonable upgrade (500-1000 used) will get you a big improvement, Take advantage of the cructhfield or safesoundhq trials.

Get a nice tube phono stage. Hagerman cornet II/III is good price used