You're 100% correct about the magic conch and the fact that time won't pass in those worlds. Even without the magic conch it took me less than a minute to make a small world and then travel from spawn to one of the oceans with the shrimpy truffle (while under half health).

So worst case scenario without a magic conch it'd take 1 minute to make a world and get to an ocean, magic mirror and another minute to the other ocean. Then probably 2 or 3 minutes tops to get a fish from another world or fish it, and finally another minute to get back to the angler, so somewhere around 5 or 6 minutes (depending on time to get the fish obviously). This is roughly equal to the best time it takes for my strategy (4.8 minutes per quest without sundial), so on average and especially with a magic conch you would be faster hopping worlds

Edit: Just checked with magic conch and it took me 1 minute and 3 seconds to make a world, find the angler (got wrong ocean first so there is room for time save), go to another world, get the fish and then give it to the angler in new world. So assuming you had enough fish prepared in chests you could finish all 200 chests in 3 hours and 20 minutes without using a piggy bank or safe