I'd personally say to not worry about Kaizo:Light or Kaizo:Hard but
1) I'm not Katerpie
2) I'm really bad at video games, so I test everything at 25% without quite knowing whether I need to, and
3) I tend to just watch videos of Kaizo levels instead of playing them myself.
With that being said, here's my advice. Warning: I hope you like reading because I'm going to give a lot.
Don't be afraid to use other people's ideas as a base as long as you make them your own. You can get a whole bunch of ideas about where to go from even just making a single jump that's roughly similar to one you saw in another hack! I've made an entire (long) room that was inspired by a single five second bit in Mario Must Die. Feel free to look at non-kaizo hacks for this too, YUMP is full to bursting with bizarre gimmicks, as are the VIP series, JUMP, and the ASMT series (I haven't watched raocow in a while so I don't know a lot about what's currently going on in the scene beyond that).
Adding on to this, you can look at speedruns with lots of breaks in them and see if you can apply those breaks in an interesting way. The worst thing about SMW speedruns is that the cape and Yoshi are everywhere, and those let you basically skip levels, but there are tons of ROM hacks and you'll find something neat somewhere.
Bosses are a good place to start with level design, honestly. They're small, they're fun, and it can even let you implement a gimmick that you only have a few ideas for in an interesting way. People need more boss fights in Kaizo hacks in general -Mario Must Die 3 had the right idea. There's a patch somewhere that lets you implement boss rushes, too, so you can even have multiple bosses in one level! (And thus automatically be a better game
If you like puzzles and want to be a jerk to the player (...of course you do, this is Kaizo), look at the glitch list. There are about 300 "useful" glitches and I haven't seen people use a lot of them in hacks (With that being said this will crank your hack up to Kaizo: Hard very quickly). Looking at levels in Tatsujin Mario: Part 1 is going to help you out with getting ideas for these (actually playing it can be hell though).
There are a LOT of custom blocks and sprites here and lion (I think) mentioned that despite playing a lot of hacks he's never even seen most of them get used once!
If you aren't planning on getting really
obnoxious with glitch abuse, using the SA-1 pack is probably a good idea (and even then you might be able to get away with it - ...I should test that). Most stuff works fine with it as long as you convert it first, and you'll get way more room to play with if you want to get wild with sprites.
Add both of those ideas and you can get a straight up bullet hell sort of thing going on sometimes. Reflecting blocks with hammers and baseballs! Pansers and Venuses and Bros, oh my!
Some sprites that might pique your interest are:
-Thermal Keys + Key Lock Blocks (Someone in the #kaizo discord posted a level using thermal keys that looked nuts).
-Revive Blocks (There are a ton of obtuse ways to kill yourself in SMW and dying sends you downwards off the screen. You can see how these might combine into something neat.)
-Punchy, Tornado*, and Chuckya* (They throw objects and Mario around and can launch things places)
*Tornado and Chuckya only launch Mario around, but you can change this...
-the various explosive sprites (There's a patch that lets you make certain blocks interact with explosions)
-Mandew's Platform Megapack (You'll go far, kid (on a platform))
-FLUDD (you can fly!)
-Disassemblies and Noob Sprites* (See * for Tornado/Chuckya. You're heading into big boy land here...)
Big Boy Land is ASM, of course. This is the nightmare mode.
ASM is...if not necessarily super easy, not as hard as you'd think it is. If you want a really good handle on it, read Ersanio's Assembly for the SNES
(The SMW one only covers the basics). While you're doing this, get an understanding of what's going on by rubber-ducking it; you comment every line of code as if you were trying to explain it to...well...a rubber duck. This might sound intimidating, but the RAM map is completely mapped out, so whenever you don't know what an address does, just look over there and you'll figure it out.
The neat thing about assembly is that it's so basic that you can literally see the 1's and 0's themselves being manipulated, which is very different from higher-level programming languages. If you're intimidated by or have had bad experiences with learning how to code, assembly's pretty different in that it's a lot more hands-on in a sense. You might have fun with it, and if you do, you might be able to get a better handle on programming in general if you're into that sort of thing!
Basically, welcome to Kaizo-land. We hope you stay!
EDIT: Please don't make the player use wall-jumps, they're really annoying and are really more of a Pit thing anyway. Despite loving weird glitches, I have an irrational hatred of these, so I might be biased.