I recommend overkill. Something American architects and engineers today don't value as highly as they should.

Overkill in my opinion is the difference between a house that will stand for 100 years and one that can stand for 500 years.
We can engineer it to the requirements of the materials and conditions as they exist now, but conditions change, materials degrade.

Especially in a situation like I see here, tying into a live tree and such, conditions will change significantly over time.