I’m also a competitive swimmer, I swim mainly fly and breaststroke like yourself and am the same height as you, but I do weigh more than you (88kg, not on my arms, ha!), and I have felt very similar reactions to swimming open water as you have described. I think there will be some water that is going to go in the suit regardless, but if it feels like it is weighing you down, then something is not right. I wear an Xterra Vortex, Medium Long suit, which is a bit tight (an arena carbon pro jammer is infinitely harder to put on however haha) but I can still get it on with minimal difficulty, and I don’t imagine it’d be too radically different from the one you have in terms of size. The size you have is most likely ok I would estimate, you may want to consider a medium for your next purchase if you want to try a different size. If you went down to a medium, I don’t imagine that it would be too radically short for you, but you don’t know for sure until you try it on (and if it is just shave your legs
).
I have found that in regards to comparing pool times to open water, best case scenario is adding on 10% of the pool time to get an open water time, which is about what you have done. I think this is normal, all my teammates that do triathlons add on at least that much time if not more from their pool times, and they are all different swimmer types (breaststroke sprint, freestyle sprint, distance free/im). I think some factors to this are the fact that: there is more disturbance in the water from not having lane ropes and waves, you do not have walls (huge difference!!!), the water being colder, every time you lift your head to sight and having to move around other swimmers makes a difference too. Also, water outside to me actually feels heavier to move through, though I don’t have a good explanation for this. Maybe it’s because my wetsuit sleeves are too tight? My next wetsuit I’m going to go sleeveless to see what it’s like, and you may want to give that a shot too. Even if the water is 57F, once you’d get up and moving past the first several hundred yards, I bet you’d be warm enough to feel fine.
Also in terms of technique, another breaststroke swimmer turned triathlete friend of mine (who has won plenty of sprints), had me shorten my stroke length (which is absolutely contradictory to what you want to do in the pool), had me swim with a higher turnover, getting the most propulsion out of the last phase of the stroke where you push through with your triceps. I was able to drop time, swim more efficiently and save more energy for the later parts of the race by doing this. I feel this is a very effective strategy for openwater swimming and would advise you to try that out as well.
I hope this helps you out, and if you end up trying any of this, let us know how it goes!