I am not familiar with ND filters at all. I’ll have to look this up.
I bought the 16-35 f4 so I could shoot real estate photos on the side. Any recommendations on a good filter? And what do they do? (I know, probably a dumb question here)...
I keep going back and forth with selling my camera and buying into another one but I dont think I’ll do that. If anything, I’ll add another camera in the future (if needed).
Is having a gimbal with me a good idea?
I think that we (you) should apply Occam's razor's principle here (I am using it in wrong way, but anyway).
Don't make things more complicated than it should be. You absolutely do not need ND filters, you don't need to sell your camera, you don't need to buy another camera.
A new camera is NOT going to magically record the beautiful video of Budapest you envision in your mind.
It is all gear and, to be honest, it doesn't matter that much. You may heard of Hollywood directors shooting movie by iPhones - the actual camera means not so much, as the overall shooting process planning.
My advice: just use your Sony, use the lens you have already, DO NOT invest in ND filters and other fancy things.
Just plan your shooting, make a simple script (will be helpful - like having 7 main chapters as 1.Flight; 2.Arrival; 3.Getting to Hotel; 4.Visiting main places; 5.Food; 6.Impressions; 7.Departure to new location).
Each chapter will be 1 min. long; there will be 5 shots (scenes) in each chapter, each scene 12 seconds long; so you have script with 35 scenes, total length 7 minuts.
Your task is now structured in a straightforward way: you need to shoot 5 nice video shots in some sort of sequence, telling the Chapter's story. Sample sequence for Flight chapter: you are departing at New Jersey airport; entering plane; having a lunch; preparing for long flight; a smiling flight attendant gives you whisky - ain't international travel fun?-, you sit near smiling Italians, who are waving to camera, you waking up at Budapest airport.
All chapters together tell an arch story of your visit to Budapest. Next story is about Prague.
And each stories together will tell a story of the European travel.
The only investment you may need is gimbal. But even gimbal is not necessary per se. If you can shoot handheld well, use your monopod, use your brain (sorry) well, you won't even need a gimbal but still will get nice stable shots using just a monopod.
So concentrate on main things and, please, forget about ND filters. They will not magically transform your video into a great story. Instead, think what you want to tell us about Budapest in your video. Is it the great river view, old Austrian buildings, incredible Hungarian girls, great food?
And the crucial thing we even not discussing here is post processing. This is arguably most important thing after video shots themselves. And it does need its own place.