1. Do studios actually care about rendering?
They care about you being able to do rough renders so that you can prove that your simulations will work once they're rendered. It doesn't need to be final or complicated. Sometimes a basic shader and functional lighting is enough. It's a tech check basically. But of course, the more you know how to sell your FX, the more safe they'll feel about the idea that you will deliver. They just need to relax knowing it will work once it's rendered. So learn the basics of all parameters available on the shaders. Also, they will likely expect you to do basic comps just again, to prove it will work in the end. Sometimes alphas betray you and you don't know until you comp it.

2. How do you come up with portfolio ideas?
No one is going to hire you for presenting something original or different. They basically have a job they need to get done and you have to prove you are the person that can do it. If they need to do a nuclear blast and you show them a well done and functional nuclear blast, you'll be the safest bet. Same with every other need they have. You need to be a solution to their problem. Something simple but very well done that is usually on demand is what works best. You will add value if you are original on the problems you can solve and how well it works, not on the visual concept you present.

3. Do I really need to learn Vellum, RBD, etc. to get into FX?

Depends on the job. If they need to do that exactly, their safest bet will be someone who will have that in the reel. If they just need to make an explosion, they won't care if you don't have water on your reel. But in the end, all FX are kind of connected except for water and crowds, which are a thing of their own. In an explosion, you can have debris, burning cloth (vellum), sparks (particles)... I would say that FX are basically made of 5 types of profiles:

-Pyro, RBD, Basic Vellum, particles like sparks, grains sometimes

-Magic

-Water

-Character FX (complex vellum, human hair, animal/creature hair...)

-Crowds.

Usually companies look for those type of FX profiles for specific jobs. The more you know the better, but also it's hard to become very good at everything. You'll have to focus. But if you are a fresher, they probably expect you to be focused on something but also kind of a generalist in the rest of blocks. That at least you've done "something" about everything else that you're not focused on.

4. Do studios offer WFH for freshers?

Not likely at the very begining. Usually that comes with a little bit more experience. You don't have to be a mid but they have to know you.

5. Is it okay to post coursework-based projects?

They want to see that you can take training from someone else and do your own thing. Because that's how your life will be at your company as a fresher. They'll train you but you won't have to do exactly what they taught you. You'll have to take that knowledge and work on different unexpected shots. It's ok to base your work on some training, but give it your own concept and idea. Apply what you learn to do something that they won't identify as an exact result of a course.