I don't think the posters original question has been answered here.
I have the same issue. When I customise the toolbar and then click save it does customise the "Quick Tools" toolbar for that document but then I need to do it every time I open a new document. How can I my customised "Quick Tools" toolbar as the default for all documents?

Please, allow me to share my custom toolbar with you, in case U find it useful.
You can use it as a template that U can edit as U wish.
Furthermore, I heard a lot of customers complaining about the right hand tool pane that takes up so much screen space. I have taken it out from my customized tool bar. I hope U enjoy it.

Here are the steps U follow to import it & edit it if U like:
1. Download the file
2. Double click it to import to Acrobat DC. But if U have Acrobat XI installed on the same machine this may not work. So, skip to step 3
3. Open Acrobat DC and click on the wrench icon on the right or click the tools tab at the top left corner.
4. Scroll to the bottom of the tools & click on the drop down arrow of the "Create custom tool" button
5. Click "Import" & navigate to the location U downloaded the customized tool bar to and import it
6. Close Acrobat & open it again
7. Go to the tools menu by clicking the "Tools" button at the top left corner
8. Scroll to the bottom and U will find the Customized tool I created. It is named "DCSet"
9. Go to the "DCSet" and click "Add". Voila! U now have it added to the tool pane on the right. But thankfully even when U click on it to add the tools set to the tool bar, the tool pane remains collapsed.
10. If U want to edit the DCSet, go to it and click the drop down arrow and select "Edit"
11. Add and remove whatever icon U with. But be careful not to add anything to the tool pane on the right, only add icons to the tool bar at the top.

Hi,

I would like to inform you about the Customize Quick Tools Feature in Acrobat DC. This is how you can easily customize all your tools in the Quick Tools list.

1) Launch any pdf
2) right click on the top toolbar and select “Customize quick Tools”
3) you can easily add/Delete/reorder the desired tools from different apps into the Main top toolbar and thus create your own customized quick tools list.

Let me know if that worked for you. or you have any other ask in this.

Thanks
Ayush Jain