Hmmm... Our designers can't edit our Custom Forms once the request is converted to a Project (due to licenses). I could have them add it to the Updates and the PM could add it to the Custom Form. Most important is that it populates on a report so the extra step is worth it. Hope my PMs agree 😀 .
Do your designers have a Work license? They should be able to edit the project custom form but you may need to put it in a section and provide the edit access on the section.