Okay you're seeing progress, that's great! What amounts of aquarium salts are you using? I do about 1-1 1/2 teaspoons per 10L bucket during a water change (only if I have removed water, otherwise you'll be increasing the salt).
You can use garden worms if they're different to nightcrawlers where you live, they might be smaller. But only if you don't use any chemical additives to the soil. You can try red wrigglers, but some axolotls wont eat them because they have a bad taste. Blanching might help
Check how much of a current the filter makes. You want as little water movement as possible. If you need to reduce it a prefilter sponge shoved into the output works wonders!
I've learnt so much from this site, I'm so glad I found it! Keep me updated on how he's going.