Is there any way for me to transfer all of the data from my LCD steam deck over to my OLED one without taking it apart and switching the internal sd?

Yes, there are several ways to do this. In increasing order of difficulty they are:

1. Swapping the SSD, although, as you said, this option is off the table.
2. Cloning the old SSD to a USB drive and then restoring that clone to the new one. This requires a USB drive big enough to hold the image as well as 1-2 USB thumb sticks to boot from and run the cloning software.
3. Manually migrating the local files over the network from the old Deck to the new Deck. This requires either using SSH/SFTP or a 3rd party utility to transfer all the files.
4. Setting up the new Deck from scratch and mirroring everything you did the first time in setting up your old Deck.

You can have it prompt you for your user/pass but one of the drawbacks is that you have to use an external keyboard since the login manager doesn’t provide an on-screen keyboard.

If you’re ok with that, then you can look into the SDDM config file in /etc/sddm.conf.d/zz-steamos-autologin.conf

For that price range, the Steam Deck is probably the best you’re going to get for a full system. The only decent upgrade is at the $700-800 range where you can get an RTX 3050 laptop.

On the other hand, if you’ve got the i7-6700 Inspiron, then you could extend the life a bit by going for a lower wattage GPU like a GTX 1630 which should be able to run on that mobo, but you should do some research on it just in case.

If you can return, then do that and you can use the funds to get the 512GB and a few games.

I’d actually argue that the installation of the 1TB SSD actually lowers the original value quite a bit since it’s been opened once and you have no way of verifying the quality without opening it up again.

So I’d drop the base value all the way down to $200 and then add the $90 SSD upgrade and would consider $300 a fair price in line with what u/tyfunk02 posted.

OTOH, if you’re selling to an unsupported country, you can easily get up to $500-600 USD minus shipping.