I would neither use walnut shells nor catappa nor oak leaf because these plants increase the electrical conductivity more than the “old” tips to lower pH like peat or alder.
I cannot measure tannins but pH and EC. Walnut a.s.o. can reduce pH to 5, peat and alder to 3.5 without boosting the EC.
On the other hand leaf is looking very nice, a difficult decision… 🙂