Only during this week did I realise what the issue was with the light leaking into the bucket and stunting the roots. My country is on lockdown, so it's impossible to get to the hardware store to buy things to help fix up my setup so I had to make do with a patch job for now - but I ran some experiments with dropping my phone (while recording video) into a spare bucket and put that inside the tent to see how much light was getting in... I put the buckets inside other buckets so there are two layers, and I wrapped the top part in foil to help protect that part. I also wrapped foil around the air hose to help prevent some light getting in.
I also upgraded to a Mars TS-1000, which probably doesn't help the light problem. But I have some more tricks up my sleeves for this weekend involving black plastic bags to help keep light out. As well as some extra nutrients to supplement the GHE Flora series.