My strumming is inconsistent

This can be a number of issues but the root cause usually comes down to discipline.

You MUST keep your hand moving!  Pauses at the top or bottom of your strum are a common problem.  If your hand stops moving, you’re doing it wrong.

A lot of times students have an easier time strumming down than up.  Other times you might strum faster when you are actually strumming the strings than when you are skipping them.  Either way, they need to be even.  Keep your hand moving at the same pace throughout your entire pattern!

I keep plucking the wrong string

Of course you do! Everyone always thinks the picking hand should be easier than the chording hand but that is simply not the case. Keep working at it. Familiarity comes with time.

My strumming sounds really harsh

This is a big one and there can be a number of reasons.

By far the most common one is holding the pick too tightly.  Now I know what you’re going to say and the answer is no, the pick will not fall out of your hand if you lighten your grip.  In fact, the pick will slip less if it has some give to it.  Holding it tightly only creates more friction which makes the sound of the strings less pleasing to the ear and the pick more likely to slide from your grip.