This looks familiar. I think you posted a similar program once, and one of the flaws I pointed out is that you're fixating on one single set/rep scheme for each exercise. But you have some good questions, so I will try to answer these.
I have modeled the rep ranges and exercises after this link. Should I be keeping everything constant at 3x8/4x8 (with the exception of some of the 5x5/4x6 compound movements)? If not, how would you tweak the rep ranges?
One of the things you should do, is to not really care about how many reps you do per set. You can get a good workout by doing 3 reps per set, and by doing 15 reps per set. The key is variation.
Personally, I like to set multiple short term goals for each of the big compound lifts. For instance, if I've managed 3 reps at 100kg bench, then I will set a goal to get 5 reps of 100kg, and 3 reps at 102.5kg. I basically have one goal for 1rep, 3rep, 5rep, 8rep, per exercise. But that's just something I do to keep things interesting.
My ratio of squats:romanian deadlifts:deadlifts is 2:2:1. Is this a good ratio?
I'm afraid that's not really possible to answer. It's going to be good for some, bad for others. There's no one answer for everyone. You're going to have to try it, and if you stagnate, mix it up.
Is there any muscle group that I am missing or hitting too much? I would like for this program to be as balanced as possible in what parts of the body it hits.
With the compound movements you have, you're pretty much set. The only thing I'm worried about is that you're doing the same exact thing week after week, which is a recipe for plateaus. Rather than adding more exercises, remember that each exercise has a multitude of different variations.
For example, the chest fly can be done in every direction from downward to upward. They should also be done past midpoint because that's not something you can typically do with the pressing exercises.
There are examples of this for pretty much every isolation exercise.
How would you recommend switching up the exercises?
For one thing, I would like to see proper squats. Preferably front squats. It's a bit trickier, and feels heavier, but it will work wonders for your quads, which you're woefully missing out on. In fact, I agree with JohnP's comment. The legs consist of so many muscles, you'll be better off dedicating an entire day to them to get it done properly. Split squats are great, but more of a secondary leg exercise.
I'm missing side-to-side movements for your legs. Very common mistake. Everything is just forwards/backwards and up/down. In order to train your adductors and abductors, there should be more side-to-side movement. Preferably in conjunction with my next point, which is...
Athletic movements! Sure, grabbing and weight and moving it up and down is what we do as weightlifters, but everything is very static. You're either standing in one spot, or laying down. I'm assuming that you want an athletic-looking body (given your current plan), but as Jeff Cavaliere so eloquently puts it; "if you want to look like an athlete, you gotta train like an athlete". And athletes don't train by standing still.
Box jumps, farmer's carry, tire flips, burpees... There are a LOT of movements you can do - some simple, some advanced - that will build muscle AND provide conditioning. They're essentially compound lifts and cardio combined.
I don't want to go on forever here, but one last thing I want to pick on is that if you're always doing the exercises in that particular order, then you're always doing the first exercise with lots of energy, and doing the last exercise pre-exhausted. Definitely mix that up.
Is doing that much cardio after hitting legs bad?
This is another impossible question. It works for some, not for others. You're going to have to try it and see if it works for you.
A program is never perfect. There's always room for improvement, but a program doesn't have to be perfect to do the job. If you focus on the things I've already mentioned, you'll have a better program, that much I can say.
I'm also sure there are good points to be made that I've neglected to mention, and if so, I'm sure there will be comments.
Feel free to ask follow-up questions. Your question is rather broad, and so the answer gets a bit lengthy too. More focused questions are good too.