Thanks for this question!
Many of the ingredients found in products designed to thicken hair or stimulate new hair growth are typically ones that fall into the medical/pharmaceutical category, whereas Just the Goods focuses on products that are more conventionally cosmetics. As such, Just the Goods does not make or sell products of this nature. However, JTG can help to reduce breakage, largely through the offering of products that can help to keep hair well moisturized.
For example, Just the Goods deep conditioning mask: https://justthegoods.net/products/vegan-oil-treatment-for-hair
Beyond this, I’d like to recommend washing hair as infrequently as possible but, when doing so, using both a moisturizing shampoo and conditioner. For example, Jack59’s Restore shampoos and conditioners: https://justthegoods.net/search?q=jack59+restore (sample sizes are available for both).
If your hair is very prone to tangling, Just the Goods’ leave in conditioner can help; https://justthegoods.net/products/vegan-leave-in-hair-conditioning-spray
One more suggestion: use a wide tooth brush or comb to minimize breakage, and to always avoid brushing your hair while it's wet.... a wide tooth comb is best in that situation, but this brush is better than most https://justthegoods.net/products/jack59-bamboo-hair-brush
I hope this is helpful!! 😊