From all indications, the RAM is user upgradable. Paying the Apple Tax on RAM makes no sense in this case.
Your use case is similar to mine, so upgrading the built in storage makes little sense either. 256GB SSD is a $0 upgrade. Do that because all of your media will be living on external drives anyway.
That leaves CPU & GPU. If you're heavy on video and can truly only do one, then make it the GPU. Both CPU and GPU upgrades seem reasonably priced though, both make sense to me.