Tactics for the Swamp (Fortress).

It's not a pity to lose Wyverns... the wood resource is usually not well-guarded... but there is still a limited amount of it...
You have to fight Goblins and Archers, of course, in the first week... but there are many neutral creatures with whom losses will be significant with such an army...
I buy Wyverns in the second week, and even not at the beginning of the week, but in the middle or at the end... My experience has shown that buying Wyverns pays off... if there is a different approach, I don't argue...

There is usually not enough wood...
If there is little wood, there are two options:
1) Build a Wyvern and try to find wood to build a Market, Guild, or Forge (the task becomes more difficult if there is no second-level building initially). This is not a bad option, but there may be problems with money and, consequently, with the army.

2) Build a Town Hall, hire all first- and second-level creatures, and go in search of wood. When building structures, there will be enough money for the army, but there may be problems with the monsters guarding the resources or the exit.

P.S. My experience has shown that buying Wyverns does not always pay off