"I feel like there must be a way to store encrypted files that, upon the presentation of a death certificate and a will, can be made available to the executor."
Presentation of the death certificate to whom? If to some online service provider....good luck. That will delay probate for many months.
Consider something simple like giving your executor access to a password protected file on onedrive or google drive. BETTER YET, if your executor has a google drive or onedrive, let THEM give you access to the file on their drive.

"My will is silent with regard to specific belongings. I’d rather give my things to my children while I’m alive to see them enjoy the gifts. Maybe when I’m older I’ll feel differently about that."
I think that's great, but please don't die unexpectedly.

Per guidance from a Trust and Estate attorney: my will gives reference to items to be gifted according to tags on the items, or photo's of the item captioned "for xxxx" The Photos file is on the executor's Google drive, password protected, shared to me...so I can modify as I wish.