I have same problem with plywood rot on my trailer sailor so am learning.

Heres what seems to be the most common advice. and im doing.

Cut all the rot out plus another 50 to 100 mm past . Because of the 3 types of rot dry rot is hardest to spot .

If thats not possible and even then as precaution use a fungicide. Ive narrowed it to boron or copper based in Boric acid or Copper Napthanate or a combination of both known as ccb.

https://static1.squarespace.com/stat...nd+Synergy.pdf

Theres 3 types of rot and wood hardners dont fix both types you have to id the rot and appropriate hardner.

Ive gone to the edge where possible but where its not if you need join a new ply piece in scarf them together ,at min 8 to1 and or put a plywood piece behind join like you do repairing jib walls (esp if you cant see it)