As long as your GC is not expired, you're fine to re-enter without the extension letter.
If you were filing now, I would advise you to file asap, as most I-751 cases are receiving their extension letters within a couple weeks of filing. I-751s are moving to online processing, so I think that's a good sign for the future.
Ultimately, I think it's best to wait and see what current I-751 processing is like in 2025.