If all the light you have is a 2 tube t5 I wouldn't expect much and it could help explain why that 6 month old plant is that small. It looks like that 6 month old also has signs of revegging.
They're both Chinese lights. The Spider is probably a little better quality but I wouldn't bet on it lasting 100,000 hours or even half that. I'm not drinking the Kool-Aid.
Probably a combination. Your water is probably high in calcium which locks out magnesium and potassium. And that soil was probably hot when you planted them and fried a little of the older leaves. What is the final ppm of the water you're using after you mix them?
If you have a couple hundred this would rock that 2x4. 100-340 watts. It's like $210 after the "boxlapse" code.
https://horticulturelightinggroup.com/collections/kits/products/320w-xl-qb-v2-led-kit
It should be fine. I usually try to do some defoliating before flip, then lollipop at day 21 and do a touchup on the 28th. But I've done it later without issues too.