The problem with "templates" is not only how to add elements to them but also what kind of file you need to export; eg. Will it be for printing or for digital use. Then what platform it needs to be used on or do the people using the templates usually have Adobe subscription or not, or are they mostly Windows or Office users, etc. There's always some part of education and how-to that needs to be added to the template or you'll miss the goal. That's especially true for Print projects; you can't really control the RGB images people will add to their layout!
For the office kind of people, I discovered that Powerpoint is one that most of them find easy to use and the .ppt file can also be opened by other software than the one in the Office Suite (eg. Open Office that is open source and free.) Unlike a .doc and/or .docx, there isn't too many compatibility issues or restrictions. It's also kind of fun to use, you can use images with transparent backgrounds and create very nice layouts that you can lock. I find Word a bit clumsy and unpredictable for this, especially if you want to use a lot of graphic elements mixed with text boxes. It's also possible to export the Powerpoint files in many useful formats, even for print-ready files. I'm not a Office fan but for beginners, it does the job and can actually offer better quality PDF than some Photoshop projects (if that matters.)
For people who can use Adobe but are not experts, they seem to be able to manage well the files created in Illustrator first, then Photoshop. Note that the Photoshop subscription can be obtained at lower cost than the whole Illustrator/Photoshop/Indesign package. So if your templates are for Photographers or small offices instead of designers, they might not have access to Illustrator. Same goes for Acrobat Pro, it's not free and not that easy to use.
Don't forget, whatever template you create, make sure you use fonts that are available to the people who will use your templates! For example, the good old system fonts.