MDF is hard on tools. Dremel is great but don't think it's gonna be the best.
It's also super dusty and crappy. I'd prefer to cut once and be done and not have to play with it too much. Depending on your dust collection it may be less of a concern for you.
Building my cabinet I had an 805mm radius to put into the cabinet side and I used a router and straight cut bit. Always do for perfect curves. Making a jig is simple enough. I always cut half the depth then second pass all the way through. Sometimes I take more than 2 passes depending on what I am doing.
I would imagine using a Dremel on 3/4 MDF would be a royal pain and expensive on bits. If you have a router use that. There are many tutorials on how to do curves etc.
If making square holes with rounded corners, one can use good double sided tape and some straight flat stock to trim out the square pattern with 90 sharp corners, for the bit to follow, and the radius of the cutter will be the radius of your corners. I used this for some hand holds and would definitely use when making a bezel for a flat screen etc.
What's cool too is that routers and a couple bits make template making a fruitful endeavor and allow you to reproduce results should you want to make another or fix an error. It's why I have a board with two handles cut out in it... Any cab I build needs some hand holds. Took 10 min to make jig and takes 5 min to make a new set of handles in a cab.
If router is out of the question, find the appropriate blade for a jig saw, and go slow. Just like sanding, the tool should be doing all the work. Once you force something one way, it compensates in another way... At least that's what my tub surround taught me lol