Dread extensions are a good decision if you want longer dreads. I extended my hair too, and i had around shoulder length hair like you. In that case, i would highly recommend you to buy real human hair extensions and dread it yourself - as soaring eagle says, it wont feel like your hair if you add on synthetic hair for example.
In my experience, if you choose real human hair, after the extensions have started to lock up together with your own hair, you cant tell the difference. Its real human hair, it will feel just like your own, and eventually, you will even forget you have extensions. Also, I didn't experience any added weight either, that was never a problem.
Extensions is also a great sulution when it comes to shrinking, If your hair is already pretty short, and you're afraid you will loose undesireable much length. As soaring eagle says, many cut off their extensions eventually, for example after the shrinking is over and your dreads have started to grow again and reached the desirable length.
And last but not least - the best time to add the extensions in (in my experience) is between the 2 and 4 months mark. If you wait much longer than 4-5 months, your dreads will probably be so tight that you will have a very hard time putting them in.
You've maybe noticed that extensions is rarely recommended, but i think thats probably becuase dreads are a natural thing, and you want as natural dreads as possible, but I don't think adding on real human hair is any less natural, it's simply for the aesthetic preference. I wouldnt add on synthetics tho, because, well - it's plastic hair, and it will feel more stiff, it will be more itchy, and feel very unatural.
it worked so well for me, and maybe it will for you too! I would definately try, and you could always removed them when you want. If you do, let me know how it turns out!