Placed file formats for print-ready PDFs

Like others already said - you can put almost anything into InDesign - and as long as color profiles are applied and are correct - there is no need to do extra work. 

The only reason to use CMYK is when you need to have full control over the output - high quality catalogs, etc. 

Flatten PSDs before importing in InDesign is not so good as you loose transparency and the possibility to use Layer Compositions. There is nothing wrong using layered PSDs. You need not to convert a PSD to TIFF except when the images are very large and PSDs would not support those dimensions

As other said, keep RGB images in RGB.

I usually flatten all my placed PSDs,

Also, an advantage of placing layered .PSDs over TIFFs is, a PSD’s layers can be controlled via InDesign’s Object>Object Layer Options...

And, placing a flattened copy of a layered Photoshop file doesn’t do anything because a placed, layered PSD always gets flattened on the PDF Export even with a preset that keeps transparency live like PDF/X-4.

A placed, layered .PSD with Object Layer Options open:

Exported to PDF/X-4, inspecting the image with Object Inspector:

Yeh no need to do any of this. 

Place what's supplied - if you are making edits then save rasters as Tiff or PSD - or if it's a Photoshop file with text layers or vector masks/shapes - then Photoshop PDF with editing capabilities

.ai is preferred over .eps

EPS are quite archaic. 

In short, you are making a ridiculous amount of extra work for yourself. In fact, you should really be placing properly tagged RGB images, not CMYK.

And to be clear, none of this is new. What you're describing is essentially a 20-year-old workflow. Do yourself a favor and talk to your printers about this.