Try pushing your fingernail into it , if it goes in easily , don't use it , too soft of a wood .
That's generally a good rule, but I think it has more to do/w how long
the wood is likely to last. Softer woods rot quicker.
Allowing for the fact that at any time if we "collect" wood from "outside"
as my wife likes to call it, we take a chance that it may contain something
which could hurt something in our tank(s).
Your obviously getting way into the very middle of it to draw out what I
think is the sap of whatever kind of plant it was.
I personally think you have boiled it enough.
It's my belief that the main purpose of boiling is to kill any disease/germs
or hidden larva in the wood. I'd say that you have done this.
But it wouldn't hurt to get this co-signed.
Actually I'd be more concerned about what I might introduce into
my tank from the water at a pet shop that I carried home a fish in.
My wood that I collect is usually far smaller than yours, but I usually just
put it right into the tank without any prep because of where I collect
from. Ouachita National Forest. This is a picture of the top of some of it.
I collect at the bottom where the streams/lakes are.