Although not a way to slow down time, it's worth mentioning that time itself freezes during the fishing mini-game in single player so you can take as long as you need to reel it in. Use that knowledge to your advantage.

Mods.

A fishing heavy day will usually results in 30 minutes run, rather than 15 or 20, which is a lot with vanilla time speed. Modding it will break the game balance.

Having to water too much crops delay your fishing schedule. Seed money delays Fiberglass rod (level 2, 1800g), which is a game changer. Not planting over 13 or 20 crops is actually a far superior choice. (which most ppl still ignore)

The Bait Maker you will unlock at level 6 allows you to catch Largemouth/Catfish frequently. Even without it, you still made a lot by practicing your Perfect catching skill, and not doing unnecessary stuff, fish until 2am.

At level 5 and 10 your fish sale price will increase. As long as you play in singleplayer, fishing time was never not enough at all.

There's a couple mods that can do this if you install SMAPI