Is my upper body strength limiting my swimming?

I swim very slow and I need to improve my speed, but despite watching a lot of videos, did some intervals, and even joined a squad, the improvement I've got is not up to my expectation. I still need over 30 minutes for 1.5 km in a long pool in last November, before the pool closed for winter maintenance. I've heard that, in order to swim fast, you have to cross the pool in fewer strokes, this indicates the efficiency of your swimming. Therefore, I have worked on my stroke length. My height is 174 cm and I can cross a 25 m pool in 18 strokes, and 50 m pool in 44 strokes, and a few less if I add some excessive glide, when I just entered the pool. But the problem is I can only do 50 m with these numbers and can't even keep my good stroke at the second length. If I try do to my longest stroke for 50 m, I cannot even continue to 200 m, and have to stop and rest to recover. If I am swimming long, I need at least 48 strokes for 50 m pool to begin with, and the number eventually goes up to around 55 strokes by the end of 1500 m (in 25 m, 21 strokes at the beginning, and 25 by the end). Furthermore, if I sacrifice some stroke length for faster stroke rate, my time actually improves and I feel less fatigued at the end of the workout. I tried to use various toys in order to see if there is a particular flaw in my stroke, but none of my toys helped me to find my problem: pull buoy - there is nearly no difference in time and stroke per length w/ or w/o it snorkel - there is nearly no difference in time and stroke per length w/ or w/o it fins - it is slightly faster with fins on, but the difference is small paddles (small size only, dare not to use anything but the smallest available) - when I use them, my stroke becomes extremely slow, but the stroke per length has only reduced very few, resulting in even a slower time using them I have bought an ankle strap and waiting for shipment, and will try it to see if it makes any difference. I also tried my favourite fist drill - without the hand, I could even fire my stroke rate higher to what I normally sustain, with shorter strokes, resulting in nearly the same speed when I swim normally. Also, I seldom feel lack of breath when I do my workouts unless I am sprinting - my 100 m top speed is faster than my sustainable speed by around 25 seconds!!!!! The perceived effort of me doing 1500 m swimming in 32 minutes is much less than doing 5 km running in 23 minutes (I run 10 km in about 49 minutes on a flat course) - but I simply can't go fast when I am swimming. After running 5 km I have to catch my breath for a few minutes, indicating full use of my aerobic capacity, but after swimming 1.5 km, my breathing returns to normal within half a minute, but my muscles are fatigued. Yesterday evening, my coach made me pull using paddles for 400 m, and I got pain in my pec for a night. I find using paddles difficult already for 200 m. Furthermore, although my cardio is not bad, I basically don't have strength, for example, even a short slope in a running cause can make me slow, leg fatigue or even cramps, therefore I don't enter running and triathlon races if the course is known to be hilly. I'm also very weak in kick sets, my quadriceps get fatigued if I kick 50 m with fins. In terms of upper body strength, I can't even do any pull-ups. Given all the above, I start to worry if it's my muscle strength rather than my technique is the major factor limiting my swimming. Basically, I feel very good if I just swim for 25 or 50 m. My stroke count for sprinting 50 m (around 50) is even less than what I am doing per length for 1500 m (I can't keep 50 unless I artificially slow it down) at a sustainable speed. Is it really possible that body strength limits the sustainability of good technique? I'm very worried if I need to add strength training into my routine because I love swimming but hate the gym.