Ditch the m/s idea and go with omnis with a center directional mic?
Another way of thinking about it is that spaced omnis with a center directional is only one figure-8 away from being MS + spaced omnis. For me the two spaced omnis are my starting point, and adding a directional mic in the center improves upon that significantly. Those three channels are the core of my setup and anything else in addition to that core-3 is gravy. From there it depends on how many channels I have available to me and what else I would like to do.
If I'm recording 4 channels, the three most attractive good options for that 4th channel are: a mono SBD feed, a figure-8 coincident with the center directional (forming a M/S pair) or a rearward facing cardioid.
I typically choose a rearward facing cardioid for more control over direct/reverberant balance, front/back dimension, audience reaction and room ambiance. I'm happy with the imaging I get from balancing the contribution of the omnis and the center directional, so control over those things is more useful to me than turning the center mic into a M/S pair for sharper forward imaging. And if I'm getting a SBD feed, that's usually going to another recorder.
After that, I'd consider adding a fig-8 to the forward facing directional for some M/S width adjustment if recording more than 4 channels. I'm looking forward to playing around with the pending Naiant figure-8 as lightweight and inexpensive way of doing that.
I'm surprised to hear you say that you find M/S and X/Y recordings to sound similar. I feel like they have the opposite issues to deal with: M/S can have tons of room ambiance at the expense of focus and clarity, and X/Y can have great localization but very poor ambiance and sense of space. I'm generalizing of course, and my experience is usually recording at a fairly significant distance. Closer to the source, I could see how the results could be much more similar, especially after some processing.
It's all in the M/S ratio. M/S with no side but all mid is 100% forward-facing Mid microphone, presumably a directional mic. If that Mid is a super or hyper (or a shotgun) it's going to have the most focus and clarity of anything you could do from that particular location. By contrast if you take an X/Y pair, and either point the mics directly to the sides 180-degrees apart, or convert a more typically angled X/Y recording to M/S and adjust the ratio to 100% Side, you're going to hear a lot of ambient room information without forward focus, the equivalent of a single sideways facing figure-8.