Your plant(s) seem to be doing fine , the stem takes a bit of time to thicken up at first and get strong , I might suggest using a silica product this will aid in boosting the stems strength . For the time being your doing well with some LST over time the stalks will fatten up naturally or like I said you can get some silica and add it to your nutrient water mix up . You could also increase the amount of wind your plant faces for addition stem training this will also fatten them up and make them sturdier . If your going to use a silica product I would consider rhino skin from Advanced nutrients . Really cost effective and its a really great product that you can use from the start to finish . I will say though that your going to need an extra hour or so of mixing time when you make your nutrient batch for your plants , I say this because silica takes time to fully dissolve in water so that your plant(s) can absorb it . I hope this information is helpful to you on your quest for glory ! Happy Growing!