After many tests in all kind of PDF presets, I couldn't figure out how to keep the text layers as vector and still have the quality of InDesign without using it. Everything I tried was rasterizing the texts. It seems like PDF/x rasterizes the layers too.
The layout needs to be created in Adobe Photoshop unfortunately, and all I have on hand is a Photoshop file with layers.
Is it possible to keep the vector quality of Photoshop text layers and objects when exporting to PDF, and get the quality of an Illustrator or InDesign file easily?
A simple trick is to prepare your Photoshop as you always do, and save a .psd file.
Then open this file in Adobe Illustrator.
You will be asked if you want to convert the layers to objects or to flatten them in a single layer. Select "convert objects".
If it doesn't work, verify if the .psd file is in 8bits.
Your text is now editable in Illustrator and appears as vectors. You can now simply do a "create outline" to make sure the fonts don't move for your print-ready or save it as a standard .PDF that you can easily optmize.
With this trick, even you get files from a friend who can only afford the Photography Plan from Adobe (Photoshop and Lightroom), you can still open the .PSD files without losing too much quality. Don't forget to ask for the fonts if the text layers are not using fonts from the TypeKit!