First off - nice looking site. Well done.
Now there are some technical things you can - Google Lighthouse complains about slow sever response and cache policy on static assets for example, but honestly those aren't anything to worry about compared to optimising your media files.
For example, I just took one of the video clips (despina_clip_06.mp), which is currently 3.6Mb, and reencoded it using some defaults in Handbrake and it came back at 700Kb. That's much more like the size I'd expect for 3 seconds of video, and honestly it looks fine. I'm sure with a bit of tweaking of pixel dimensions and frame rates and so on you could get it down even further.
The same goes for the images. I just took a sample image that was 760kb, and after resaving it and running it through ImageOptim it came back as 44kb. You'll find that if you're using double size images like this you can actually apply quite a lot of compression and still get good looking images.
I've attached a side by side comparision ... although obvioustly that's been compressed so take it with a pinch of salt.
I know that designers (and I suspect your film makers) will want to keep quality as high as possible; That might be noticeable on their fancy Macs with 4K displays but most users won't notice the difference in quality but will definitely notice the difference in download speed. You can always go ahead and recompress the files and see if they notice (bet they wont...).
s.