Hi Velcro, perhaps I can help. I have 2 mirrorless, one DLSR and of course, phone and iPad.
Mirrorless is all I shoot with underwater (diving) in a housing. It's also what I love shooting corals with. I am shooting with an Olympus OMD-em-1 or an Olympus Pen-f and a 60mm macro lens. A fixed (or prime) lens has one focal length, eg. 60mm. All of the mirrorless at a decent level (like Olympus) have the ability to shoot in manual mode and to set a custom white balance.
Here is a shot I took with my Olympus, 60mm in my tank. Now, in the interest of full disclosure this photo is a 'stack' of photos each using a slightly different focus plane, so that I can shoot with the aperture wide open but still get the focus (depth of field) thru the shot.
A great book on photography (easy and comprehensive) is written by Bryan Pedersen, called understanding exposure. I may have mis-spelled his last name! Have fun shopping and remember, it's not the camera, it's the photographer. A mirrorless is a great choice and there are wide variety of excellent ones to choose from. Check out blue water photo (it's a store/web site) - although you are not necessarily interested in diving, they explain the pros/cons of a variety of mirrorless (and other cameras) and do have a lot to share about macro.
debby