I don't have any answers to give you, but I want to say that this is a really loving and beautiful thing you're trying to do.
We cannot rule out she wants to spend quality time with her partner instead of spending time in a recording studio, so that, if the worst outcome comes, her husband can remind her of what she lost.
She can, but she might not. A lot of that depends on how he presents the idea to her -- it might seem like something that's important to him.
Honestly there is no doubt a very large emotional/personal side of this, irrespective of who's idea it is and who supports it.
Technology isn't the solution for all problems and challenges in life.
But good lord, sometimes trying to get technical help on the Internet turns into this rabbithole of people who are specifically looking for ways not to be helpful. "Did you really want that?" "Did you consider alternatives?" "What you really have is an XY problem."
#1 item from https://www.molfar.io/blog/yc-questions
Sometimes a cigar is just a cigar.
He then goes on to say "My ideal would be an open source 'deepfake toolkit' that allows me to provide pre-recorded samples of her speech and then TTS in her voice."
That sounds like wanting to capture and simulate someone's voice.
Literally the only person on this thread who knows anything about the OP's wife is the OP. Everyone else sharing an opinion on "the emotional side of this" is vocally ignorant.
I'd make no presumption, good or bad, about the their relationship dynamic, however.
And it's particularly useless when your worries are about a situation which does not concern you and which you are almost completely ignorant about.
The sentiment is admirable, but it's a lot of work considering that the probability of a negative outcome is very low.
I'm not sure there's a correlation to other senses, I can't see for my future self or move on his behalf. I suppose there are things I would want to taste or smell if I was going to lose those senses, but those are experiences for me, not things I'd use to communicate with loved ones.
After losing my voice in an accident, I'd be willing to spend many, many hours transcribing my own speech in the handful of scratchy family videos, voicemails, and phone logs of ordinary conversations. If I could spend a couple days prior to the event reading some books, a TTS training corpus, or anniversary/birthday/wedding/etc greetings and congratulations into a microphone and have a personal text-to-speech voice I'd be all over that.
It would be a little weird if someone else used it as their narrator, but that's not OP's goal.
Speaking of recording books and training corpuses, my grandparents (who have their voices) got a special kind of joy from reading children's books that they once read to me and that I once heard as a child to their new grandson. OP, if you and your wife have or might have kids (and she can handle it emotionally), it might be nice to record video/audio of reading children's books to future grandchildren. Even if your future grandchild knows that grandma can't read books out loud, I'd bet Grandma would be happy to silently turn the pages for a toddler on her lap until those digital recordings got worn and scratchy like an old VHS.
Recording audio and then choosing not to use it later is fine.
Not recording it because I don't want it right now... maybe fine? maybe sad.
This is less of a problem with modern high-quality mics than it was, say, with answering machines 30 years ago. Your voice might still sound not exactly the same, but it hopefully shouldn't be unbearably grating either.
It's because reproduced audio doesn't have the bass the same as you hearing it conducted through your jawbone (though of course this will sound too bassy to everyone else!)