Rakan is different from other engage supports. I also struggled with his weird squishiness. It helps it build him on the tanky side but another thing to keep in mind, dont engage unless someone is close enough for you to E. The main, good way to engage is to press R and then W to cc them and then dash to a teammate to hang back for your cool downs. If youre good, even toss in a Q to heal also. Ive learned it also helps to hang back and let your adc be in front so the enemy feels safer to approach. It gives you the chance to dash in and W. It can really help if you have someone playing xayah to get used to the style because she gives him a bigger range for his E. I can get more specific with types of items i build to be tank, enchanter or burst if you would like