It may not be what you want to hear, but for that budget I'd strongly consider staying where you are. The mics suggested are maybe a little better than what you have, but they aren't a LOT better. If your goal is to move to a rig that really competes as an HQ open rig (but also is nice and compact) the real next budget option are AKG actives, which I think are going to push you past $600 all-in. Beyond that of course are Schoeps, DPAs, etc.
I went through the same thing in earlier years making lots of little incremental upgrades and all it caused me to do was lose money and spend a lot of time learning how to use gear that was only a little better than what I already had. If I were you, keep making great recordings with what you have, maybe invest in a nicer stand and mounts or whatever if you want a sturdier open rig, and save your coin for an AKG active rig, or at least Busman BSC2.