the splitters will be to get the main stage mics both to front of house and to our in ear mixer. Most mics going into it will be sm58's or 57's.
Ok, cool, I get what you need.
So what you want is to build a "Mic Splitter", I can help on this subject as I work in Live Sound concerts every week and for many years.
For "Mic Splitting" or a "Mic Splitter" you don't need transformers at all, your first idea of a straight non-transformer Mic Split or "Mult" is a good idea and it's actually common use in Live Sound.
There are of course high end active mic splitters, and also mic splitters with isolated transformer outputs but for most applications a straight passive split works perfectly.
Most of the Splitters I get and use in Live sound are just passive boxes with only cabling and XLR connectors and always work well and in those scenarios up until this day I never had a problem of it not having isolation. I use one of those type of splitters regularly.
So my advice is, ditch the transformer idea and just buy rack panels with XLR holes pre-drillerd into it, and buy all the connectors and start soldering everything thing up.
One Female XLR for Mic in connected to two male XLRs for 2 outputs of the same source.
I just want to make sure that the transformer I have found will work to isolate the output to the IEM mixer without and major signal loss or distortion.
The problem with audio transformers is that budget transformers will do more harm in this scenario than good, they will provide isolation for sure, but a cheap transformer will change the sound, will have limited frequency response will have distortion at lower frequencies.
For these duties if you want transformers with less "signal loss or distortion" you would have to get higher end transformers from Lundahl, Sowter, Jensen or Cinemag, but the price for each transformer will be quite high, from 65€ to 150€ each.
And more important, as I told before, trust me, I have many experience in this area, you don't need transformers for a Live Sound Mic Split, just do passive mults and you will be fine, happy and less broke.