Can't update my Nuvi 2597LMT, won't go into update mode

You say you just used the cable to update an “older nuvi 2595LMT with no issues.  So my first question is do you have both the 2597LMT and the 2595LMT defined in GE as separate devices?  If so do both devices have microSD memory cards installed?  The current maps will need a memory card.

No, I was setting both of them up on that PC so I only have the 2595 on Express right now. And I don't have a MicroSD card yet, was going to get one, I updated all the other software on the 2595 as there was several updates for it on top of the maps and was going to do the map update once I got a MicroSD card for it.

Regardless, said 2595 as soon as I plug it into the PC is showing that connected icon on it's screen and recognized by the PC, the 2597 is just continuing to boot as if I just plugged it into power and not a PC.

It's old and written for Windows 7, but you might try the steps at garmin mass storage setting and changing it out of mtp mode.  

Something similar, and specific to the 2597 can be found at forums.garmin.com/.../garmin-2597-not-recognized-by-desktop-or-laptop

Not sure what you mean by update mode as there isn't a specific update mode. Sometimes Windows has issues with mtp devices. Do as suggested and try switching it to mass storage mode.

Didn't work, it still acts exactly as it did before, the error about the cable not working with traffic while it just boots up normally and the computer not seeing any device plugged into it.

Whatever that mode is called where after the Garmin logo instead of booting into the main menu it displays a little orange icon of a GPS plugged into a computer, it just powers on normally as if I just plugged it into power and not a PC. Also I would like to note that the error message about the cable not being suitable for traffic only appears when it's plugged into a PC, if I take that same cable and plug it into a charger it does not complain about the traffic feature.

Just to make absolutely sure it's not the cable or the PC I tried that same cable and that older 2595 in another PC, this time a Laptop that does not even have Windows installed, much less Garmin Express, but is running Linux, and the 2595 still functioned as it should where instead of the main menu it displayed a yellow icon of it being plugged into a PC, it even recognized when it was unplugged by turning the icon grey.

Tried again on that same laptop with the 2597, and same thing, it just boots up normally as if I plugged it into power with that "I agree" screen then the warning about the cable not being suitable for traffic before going to the main menu instead of recognizing it's plugged into a PC and going into "update mode" or MTP mode or whatever that mode where you can update it is called.

So this is clearly not the computer, Garmin Express, or the cable but the 2597 itself that seems to not recognize it's being plugged into a PC. And I have no idea what to do about this, any ideas? Forcing MTP in that developer menu didn't seem to make any difference.

Strange!  Do you know or can you determine if the Windows 10 laptop has a USB 3.0 port?  USB 1.0 and USB 2.0 ports can only deliver 500 mA whereas the USB 3.0 can deliver 900 mA..  Use the USB 3.0 port.  Most laptops of the last 5 to 8 years may only have USB 3.0 or better and that is good.  Verify the 2597 is fully charged.  If need be leave it connected to a wall type charger for 6 to 8 hours. 

How soon to get that microSD card (or cards) and retesting with it?  Both devices will need it for the current maps.

Hopefully you mean forcing mass storage and not mtp mode? You may have a faulty port on the nuvi, given that you've tried a cable that works with other devices.

I am not sure which ports on the laptop are 3.0 but I know I tried both the 3.0 and 2.0 ports on the desktop. Also, since the USB-C end of the cable is Mini-USB wouldn't it be limited to USB 2.0 specs and power anyway? And yes, I left the 2597 to charge overnight before I attempted updating it. And I can get cards soon, but they are worthless if I can't get it to update in the first place.

Yeah, I went to the MTP menu and set it to Mass Storage Mode.