There are kits to replace old 2/3 handle setups with a single handled pressure balanced controls. The work can all be done from the front and it has a large trim plate to cover the holes. This requires someone that is very good with plumbing work and can do soildering in a very tight space. And if you have access from the back then it won't be too hard. But a quick google I could not find them. You would need to try a good plumbing supply house.
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