Technical help with replacing Windows desktop hard drive [View all]

Hello. Thank you in advance for any help. I know just enough to be really dangerous and probably sound silly:

I have a Dell desktop that's a few years old. The boot drive (C) is only 113MB and Windows 11 has devoured it and is suffocating. I boot up and almost immediately get messages about not having any drive space. Or, I boot with 2-300 MB available and then watch it slowly whittle away. Then other programs freak out and start eating RAM. I have a 1TB D drive with plenty of room and have moved every program and file that I can there (OneDrive, all of my files, and all of the programs that would install there.) I've also run all of the cleanups, including older restore points and all the temp files. It feels like Windows has just swallowed the whole drive. Edited to add I've also paused updates in case that's what was gumming up the works.

Yesterday I got ahold of some bad specs that had me buy a 2.5" SATA drive to replace the C. I cloned it and opened the case only to find that the C drive is actually the stick type. (M.2 2280 SATA, I think.)

So, I'm not sure what to do next:

1. Drop the money to replace that drive plus whatever I'll need to clone it? Is the $150 or so I've already spent sunk?
2. Is it possible for someone competent to replace the M.2 with a 2.5" and make that work? It's already cloned and I know it'll boot (spoiler alert: before I knew it was wrong, I'd actually unplugged the D:/ to plug in the new drive. ugh.)
3. Just buy a new computer because why put more money into an old budget Dell?
4. A fourth way?