Hey, first of all congrats if you made it that far, especially if it's your introduction to the genre!
You're pretty much only missing the challenge trophies. For the 300 challenges done simply do the first few ones of every character, that shouldn't be any problem at all, I actually got that trophy just playing half of the cast
And well, actually the only platinum breaker in this game is by far beating all the challenges for one character, that definitely takes some skill.
I did it myself with Shadow Labrys, she's probably the easiest one of the standard cast, but the last 5 challenges are kinda tricky, especially since you have to figure out really well the timing with her Persona.
You should try her and see how it goes for you (some people just really hate puppeteer kind of characters), but another great option would be to get the Margaret DLC, she's arguably the easiest one by far.
Anyways, first try to get 300 challenges done, which shouldn't be that tough, and it will help you to get a better understanding of the mechanics and then get ready to choose which character you will go with for the 100% completion (being S. Labrys and Margaret the best ones)
It's hard to explain the fighting games stuff, I don't remember exactly how good the tutorial in this game was, but you can try to check it again, similar games like UNIST or BlazBlue have amazing ones that go into deep detail in every aspect of the fighting games.
You don't have to do any dash cancel for the combo challenges in this game (at least with the easiest characters) but the idea of a Jump Cancel is just that, to cancel an attack with a jump so you can continue doing other stuff instead of finishing the animation of that move. It takes a while to get used to it, but you should be performing certain move and then just jumping to continue with the rest of the combo, but again, it's all about the proper timing, you have to give it some time so your character jumps into the air and then continue the combo, not just smashing
Remember that for most of the challenges you can just put the dummy in the corner, which makes everything easier to connect, and if you're doing a jump cancel requiered challenge there you can just jump straight (since the dummy is in the corner and can't go farther) instead of jumping into his direction which is harder for most players.
Also, if you're playing with a controller I personally use the dpad instead of the joysticks, but that's up to you.
Just keep trying and eventually you will get most of the stuff without even realizing, we all had that time when we started playing fighting games, good luck! ^^