You already have way more mics than I expected. To call the SM 58 and 57 the standard of mediocre means that you are putting way too much hope in a mic making some kind of magic, in my opinion.

Both of those mics are outstanding and very often do way better than more expensive condensers in a home studio situation, where control of reflections is not really well executed for the most part. They are much more forgiving than most cardioid condenser mics because of their better off-axis rejection.

The list of excellent acoustic records made with these mics - and entire albums, is well known. That probably doesn't need to be rehashed.

What does your anti-reflection absorption situation look like in the room? I would concentrate there before buying another mic. Especially on a limited budget.

As for the mid-side mic, you should be able to use a matrix in your DAW to resolve this to pure stereo. https://www.voxengo.com/product/msed/

Those are just my opinions, but I would say you have plenty of mics to work with and tweak - but it does depend somewhat on your room - all the more-so with a less-cardioid patterned condenser.

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