Yeah catching bigger fish needs a lot of proficiency, steel rod CAN catch medium (30k fish) but only with good RNG, Springy catches 30k's with ease but has trouble with 37k, allthough it is possible.
Yeah the fishes despawn after like 7 minutes and they all have hp regen; Small fish has 18k hp and regens 25 or something hp per 2 seconds, Medium fish 30k hp regens 70hp per 2 sec i believe and Large has 37k hp and regens 100hp per 2 sec.
It's satisfying to catch large ones but they often slip out of my grasp, and i dont really want to upgrade to the gilt fishing rod yet (30k prof rod)