@reload2024 Yup, managing EDID is fun. The high-end media servers (disguise, Pixera, GreenHippo) all manage these with additional hardware and software (one company uses a mini Raspberry Pi hidden away in their cases just to manage EDID).
It sounds like you need/should keep the Mosaic on then. I would use the tools within Isadora to achieve what you need. Might be worth making a template to map out the outputs - if that makes sense? All you are doing really is managing the pixels and where they go. Keep us posted, happy to help if/when I can.