Its odd trying to get a computer to install an os to a usb in the first place. Why even bother?
Optiplex7020 is UEFI so I can see it not booting the MBR installation much less very few IDE style motherboards even booted from a usb which wasn't sorted out entirely in the field until 2005 and some motherboard manufacturers didn't even bother posting a bios update on it because CD ROMS were the choice of install media until around 2009. Unless you actually have USB Hard drive boot as an option its not going to try it. Much less you have to make sure the partition type (GPT vs MBR) is set up prior to it. Otherwise, its just going to format and install on the usb device as a MBR type which UEFI machines don't normally boot MBR and some can not be modified to do so.
But as far as trying to use all of the controller ports for the NAS and having a separate boot device, It would be easier to do that with a card than the USB. Because USB boot options are not the greatest on older hardware (if it works). What is your slot types available to you? Because its going to be easier to find a card that will boot a SSD drive on that machine than set up a usb boot system that may not be possible because of the bios.
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