Parts of 3-4 week old roots are weak and slim at some points

Could be classic root rot. Caused by your res temperature being too high, light leaks, foreign debris etc. My advice would be to find where you went wrong by figuring out what you did right. The tote looks dark to me so no light leaks. if the leaves look healthy your nutrients should be good. for temperature, I would freeze Gatorade gallon containers full of water and just pop the lid to sink one into the water to keep it cool. then I would just swap them out throughout the day. Water coolers are expensive and I didn't bother until I felt I needed one for RDWC. If I ever have any root issues I use Hydroguard and it cleans everything up. Change out your water and quickly scrub the tote cleanish to have a nice water change. Good luck!