I could most likely get away with 128 or 256, but there are times like vacations where I load up the iPad with movies and books to read so the extra space is valuable.
How many books do you read on vacation? You'd have to load up an awful lot of books before you'd have to start worrying about running low on storage space.
FWIW, I just had to order a new MBA for my wife. She had 256GB of storage on her old MBA, but was running a bit tight on storage, and I was contemplating whether I needed to get 512GB on the new one. But after digging into her storage usage, I saw that most of it was from Photos/Videos, so I decided to up our family iCloud storage plan to 2TB and switch her Photos app's iCloud settings to "Optimize Mac Storage", which freed up a huge chunk of storage.
Her other big usage is "Documents" but I think a lot of that may actually be iMovies projects. So my next decision was to purchase an external SSD drive (I haven't purchased it yet, but likely getting a Samsung T7 500GB for about $129), with the thinking that she can offload her iMovie projects onto that drive and further minimize her MBA storage needs.
Getting back on-topic...if you plan to edit movies on your iPad, using an external drive for storing files can help, but isn't as good as it is with a MacBook, because you can't edit *directly* on the external drive, but hopefully this will be improved in an upcoming version of iPad OS (I'm assuming it's an OS limitation and not something that a 3rd party developer has the ability to handle).
The OP will have to consider their own specific needs/requirements, of course. As for me, I'm also in the market to buy a 12.9" model for my daughter, and will probably go with the base (128GB) 2020 model. She already has a 2018 (?) MBP 13 and is anticipating using the iPad primarily for graphic design/illustration purposes, where much of those files can also be stored in the cloud. I suspect that she may find herself gravitating towards using the iPad more than her MBP, but even if that happens, I think that 128GB will be sufficient for her needs, as most of her TV/movie watching is via streaming services.