(Partially Solved) Debian Testing Only Booting with nomodeset (nvidia issue? kernel issue? unsure)

So, I've searched around for answers that can help me, and I don't think I've found a page that has a solution that works for me. I really hope I'm not asking a duplicate, somehow, and just missed it. (Note about edit: off-by-one error fixed in Debian versions.) PARTIAL-SOLVE EDIT: Turns out I'm a little bit stupid. I forgot to un-blacklist the nouveau driver (I had created /etc/modprobe.d/blacklist-nouevau.conf when figuring out the nvidia driver install long, long ago), and just commented out the line that said blacklist nouveau and options nouveau modeset=0. I now have a much more reasonably-scaled GUI in Gnome, so that much is behaving as expected. It doesn't get me the use of my graphics card on Debian, but maybe that's an upcoming fix. Thanks for those who responded; if anyone has any idea how to solve the graphics driver part, I'm still looking to solve that portion, but at least I can do (some) work, again! The TL;DR version of this is that I upgraded to Debian 13 (trixie) from Debian 12 (bookworm), and now no longer have a GNOME GUI without adding nomodeset to grub, which makes it very undesirable to do my day-to-day work, and I'm trying to find solutions. I'm working on older hardware (12-year-old CPU and GPU), and am having trouble either using the nouveau driver to solve the problem and getting rid of the nvidia-legacy-390xx-driver or getting the proprietary driver working again. Here is the output of neofetch that gives my kernel, hardware specs, etc:

OS: Debian GNU/Linux trixie/sid x86_64 
Kernel: 6.3.0-1-amd64 
Uptime: 27 mins 
Packages: 4359 (dpkg) 
Shell: bash 5.2.15 
Resolution: 640x480 
DE: GNOME 
WM: Mutter 
WM Theme: Adwaita 
Theme: Adwaita-dark [GTK2/3] 
Icons: gnome [GTK2/3] 
Terminal: gnome-terminal 
CPU: Intel i5-2500K (4) @ 3.700GHz 
GPU: NVIDIA GeForce GTX 560 Ti 
GPU: Intel 2nd Generation Core Process 
Memory: 3484MiB / 15903MiB

I also have full-disk encryption enabled through LUKS, and am dual-booting alongside Windows 7. (I don't really use Windows except for gaming anymore, so I don't think that's related.) I tried booting into Debian a couple days ago after the first time in a long time. At that time, it was still Debian bookworm. Prior to upgrading, I had noticed a behavior where, after booting past grub, the cursor ("_") screen would appear, and then go dark. If I blindly entered my LUKS password and gave it a moment, I would hear my hardware work on something and GNOME would eventually load, and I could proceed as normal. Since I had not updated in a very long time (months), I ran updates, noted that I'd be updating to Debian 12, and then shut down. When I came back the next day, I now could no longer boot into GNOME, and was met with a black screen. I tried entering my LUKS password, and again, I can hear the hardware work on something, but nothing actually changes on the screen. Trying to change to a different tty does not yield any command prompt, and I have to reboot to get back to grub and set the nomodeset flag to get into any kind of GUI. I thought perhaps there was an Nvidia issue, so I decided to try to remove the Nvidia driver and go back to nouveau as a temporary solution. This did not prove fruitful; I can now only get to a GUI of any kind past boot when enabling "nomodeset" in the grub menu. I did try to reinstall the proprietary Nvidia drivers, thinking that perhaps they just needed to be reconfigured after the update, after all. However, there, too, I ran into trouble: I know that I am supposed to be using the nvidia-legacy-390xx-driver package, because that is what is supported and documented on the [Debian Wiki page](https://wiki.debian.org/NvidiaGraphicsDrivers) . I also noticed, though, that when I run nvidia-detect, I get an error that I have not seen, before:

Detected NVIDIA GPUs:
01:00.0 VGA compatible controller : NVIDIA Corporation GF114 [GeForce GTX 560 Ti] [10de:1200] (rev a1)

Checking card:  NVIDIA Corporation GF114 [GeForce GTX 560 Ti] (rev a1)
Uh oh. Failed to identify your Debian suite.

I also check if I can even install the driver package itself without needing to verify nvidia-detect, or even just to search the repository for the package, I get something even more baffling, to me

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package nvidia-legacy-390xx-driver

Sorting... Done
Full Text Search... Done
nvidia-legacy-check/testing 525.116.04-1 amd64
  check for NVIDIA GPUs requiring a legacy driver

I tried editing /etc/apt/sources.list to include bullseye-backports main contrib non-free-firmware, and then I was able to find the driver package. When I tried to install the legacy driver, however, a bunch of packages could not be installed because of a dependency issue on xserver-xorg-core, which needed to be version < 2:1.20.99 but had been upgraded to version 2:21.1.7-3, which is the current version of that package. As a result, I can't even try to re-install the proprietary legacy driver and see if that will resolve the issue. I'm not sure where to go, from here. Does anyone have any idea how to solve this, either by fixing the nouveau side or by trying to get the legacy-390xx-driver reinstalled? Thanks for any help you folks can offer!