You say 1st to 2nd, but can you downshift from 2nd to 1st while underway without any grinding? Grinding would, I think, be conclusive for a bad synchro. No grinding...I'd want to check every other possibility (even knowing that the experts here are indeed experts).
The workshop manual lists four possible sources and actions:
1. Loose powertrain/drivetrain mounting bolts. Tighten bolts.
2. Pilot bearing. Install a new bearing.
3. Internal component damage or wear. Install a new transmission.
4. Gear teeth worn or damaged. Install a new transmission.
Considering the magnitude of 3 and 4, I'd want to be very sure about 1 and 2.
If it was my car and I wasn't going to sell it, I would take an action that is not in the workshop manual: Don't decelerate in 1st gear! Meanwhile, be on the lookout for a used trans to have in case your trans goes further south. Hard to imagine a spare trans is ever going to lose value.