I used both MOPS and MES buffers for a dimeric protein of 20kDa(totally 40kDa) on the same type of gels you are using. My experience is MES has better separation for proteins around 30kDa.

Here is a separation chart for Bolt gels:

https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-gel-electrophoresis/protein-gels/bolt-bis-tris-plus-gels/bolt-bis-tris-gel-performance-data.html?open=migration#migration

I think I would choose a 12% gel with MOPS running buffer.

Try it with simple 14% Gel and normal Glycerol running buffer

or 10-20% Gradient gel

Here I found a interesting publication

http://www.pnas.org/content/110/39/15668

Maybe helps. Good luck

Thank you very much for your answers!

Tris -glycine 4-20 percent gel should separate nicely, and I got good mass spec readings from the gel slice.