Thing is, I'll have to get 2 frame perfect tricks in a row (you can check it out here: https://www.speedrun.com/diwo4/thread/bschv/1#vsymq).

Any tips as to how I should practice getting that, or should I rather just pray till it eventually happens?

Look for visual ques. That's all I have to offer.

Generally it is very useful to have an indication on whether you were too early or too late for any of the frame perfect inputs, which should be the first thing you should figure out. After that yeah you should practice the timing just a lot until you have it in muscle memory. If there is something restricting you to try the trick multiple times in a row to practice it ( like health ammo or something ), then using savestates on emulator or cheatcodes may be a better option for practice. Also does this game run on 30 or 60 fps ? If it's 30, then it should be very doable. If it's 60, then well have fun practicing a lot.

Using emulator to practice can help a LOT with muscle memory of movement, but can potentially hurt visual muscle memory, due to emulator latency. I've been digging into solving input/display lag with emulators; hopefully I'll learn something good soon.

How do I know if the game runs at 60 or 30 (without anyone telling me, that is)?

regarding Digimon World 4, it seems to be 60 fps. There are various methods to get the framerate of a game from using google up to measuring it yourself with an ingame timer or just basic movement and comparing on emulator or just by looking of smooth the video looks.