I just discovered a similar issue on our Catalina 25. My chainplate setup is slightly different, anchoring to the bulkhead with four bolts. The bottom two were fine (solid bulkhead wood) but the wood around the top two was soft. The worst intrusion and rot was above a cutout where the PO had installed a radio.
After quite a bit of research, an experienced sailor buddy agreed that I didn’t need (or want) to replace the entire bulkhead. Instead, we thoroughly cleaned out the rotted wood on the head side, leaving a thin veneer on the cabin side of the bulkhead. We then epoxied in two clean pieces of ply. We reamed out and filled the two “soft” holes with epoxy, also. When the rain stops, we’ll go out and drill new holes and rebolt it, finish off the rough spots and do a little stain touch up.
It may not be pretty, but the worst part is on the head side and mostly unnoticeable. And it is strong as heck. Lastly we’ll rebed the chainplates, which I obviously should have done sooner.