I am not aware of TRS mono plugs, please have a read here: https://www.sweetwater.com/sweetcare/articles/whats-the-difference-between-ts-and-trs-cables/.
I do have the M4 and I don't remember having issues with the TRS front inputs. I've also wrote a teardown article here: https://www.audiosciencereview.com/...analysis-and-few-in-house-measurements.17819/.
Also, Motu states that front jacks are TRS: https://cdn-data.motu.com/manuals/usb-c-audio/M_Series_User_Guide.pdf at page 25, so I am not sure I fully understand the issue you're facing with this interface. It has similar front jacks like the Focusrite and should work the same way.
I do have the M4 and I don't remember having issues with the TRS front inputs. I've also wrote a teardown article here: https://www.audiosciencereview.com/...analysis-and-few-in-house-measurements.17819/.
Also, Motu states that front jacks are TRS: https://cdn-data.motu.com/manuals/usb-c-audio/M_Series_User_Guide.pdf at page 25, so I am not sure I fully understand the issue you're facing with this interface. It has similar front jacks like the Focusrite and should work the same way.