One of the reasons your plants are short and have tight nodes (bushy) is that you're vegging them under those blurple LED's. I use them for vegging as well, I like the tight nodes and bushes before flowering. Two main factors will stretch them more, but not under those blurple LED's which probably don't flower very well anyhow. Heat, and even more so, a spectrum and colour (kelvin) change between veg and flowering lights, the greater the difference between your veg/flowering lights, the more stretch you'll get. We just had a good discussion in another forum on this spectrum/colour change between veg/flower and found some science to back it up.
Here's why I don't mind these blurple LED panels for vegging. I train/veg them with tight nodes and lower temps, then when I switch to CMH for flowering they explode in the stretch period, they open up at that point and stretch like mofo's. Here's a batch I just threw into flowering with 10 days between each pic as an example: