The first way is if you want to focus on other things and just do the quests in 1-2 minutes. (You need only 1 character).

First of all you need to get a world where you can get anywhere you want.

Get the best fishing gear and max accessories possible:

Angler Tackle Bag

Angler Earring

High Test Fishing Line

Lavaproof Tackle Bag

Angler Earring

Fishing Bobber

Angler set

Good baits

Golden Fishing Rod

Depth Meter

Gold/Platinum watch

(Bottomless Water Bucket and Drill Containment Unit if you will make the world yourself)

Minion to protect yourself (Stardust Dragon Staff)


Fishing potions:

Sonar potion

Fishing potion

Chum buckets

Gummy Worm (perm boost)

(with help of user Polaris): Sitting on a toilet while fishing: +5 fishing power.
Floating using Inner Tube / Mollusk Whistle while fishing: +5 fishing power. (Obviously can't float and sit on a toilet at the same time, pick just one of these things).
Tipsy debuff grants +5 fishing power.

This should give you maximum fishing power, but you are not required to have maximum possible power tho.

Also you can get shell phone/or demon conch and ocean conch. Get Angler to your spawn point, get a bed and go fishing, after you get a quest fish you go to bed and skip time, also you can get Enchanted Sundial and use it one time a game week (once in 168 minutes), and repeat the cycle.

Also, amount of water should be 300<blocks for max efficiency.

Second way is you get a new world find Angler make multiple characters 10-40 or more. Get the gear from the first way, start from game time 4:30 AM, fish for a quest fish and put it in a Piggy Bank and repeat the cycle 40 times, once you did that get to the Angler, give him the quest fish, put other 39 in 39 chests and join from every other character to give the fish to the Angler.

Hope my guide helped, if you have questions or/and correct me tell me in the comments.
If anyone could, please donate some steam points to me