Kai

Boiling before it goes in does more in an hour than a week of cold soaking. The temperature is doing the work.

Third option and I ended up here after fighting it for a year: tannins lower pH slightly and have mild antibacterial properties, so a tank full of them is closer to what a lot of soft-water fish evolved in.

The tea colour is only a problem if you have decided it is. Shrimp and most tetras do visibly better in it.