Tip: When determining a time limit, remember that players won't be as familiar with the level as you are. If you normally complete the level with the timer around 100, others might run out of time on their first try.
I really want to get into hacking. I've loved making Mario Maker levels but I just want to go that extra step further and hack the game myself instead of being locked into what Nintendo wants me to make. I've read some basic tutorials with ASM, made some basic blocks, hijacks, etc. But what should I do to start?
It kind of depends on what you're after, honestly. I would say to take it one step at a time. Get familiar with how Lunar Magic works and make a few levels; then try inserting other things like ExGFX, blocks, sprites, and patches; then if you want, try making your own resources (which you have mentioned doing already). Make a short hack if you want, a longer one, or just design random levels until you feel like doing something else.