I mean, our stomach acid is hydrocloric acid. Just ours isn't nearly as concentrated.

Vulture pH is closer to 0.2, ours is closer to 2.

it's 10x more acidic than ours, so yeah, we would need to probably refine the entire digestive system

Ethics are a big hurdle, but I wouldn't call it the biggest. Mengele was a conscienceless monster who played at pseudo science.

The biggest hurdle to gene editing is that it's fucking hard. Even Mengele V2.0 with no morals and a stable of daily test subjects wouldn't lead to a 'mastery' that lets you merely select for your changes to only include digesting lignin and cellulose.

Seriously. It's really hard to even model within acceptable error margins, what the results of gene therapy will be. The potential for disastrous results also only goes up the more complex the interactions you're trying to affect.

At current time, even some of the best recorded attempts which are decidedly positively quality of life altering (e.g. [emphasis mine] https://www.science.org/content/article/gene-editing-therapy-made-just-6-months-helps-baby-life-threatening-disease) still come with great caveats:

The boy, first treated in February at just 7 months old, still needs a special diet and medication; the experimental therapy alone isn’t enough to prevent a dangerous buildup of ammonia in his blood caused by a faulty gene for a key liver enzyme. Still, the swift development of a gene editor that appears to have repaired the defect in some of his liver cells is a landmark demonstration of a personalized approach that has tantalized rare disease researchers.

Meanwhile, even renowned institutions starting from best intentions, get it horribly wrong, because the interactions are just that hard to model (e.g. [emphasis mine] https://www.science.org/content/article/exclusive-death-girl-chinese-gene-editing-trial-was-never-made-public):

That vague language glossed over tragedy: Seven days after the girl’s medical team infused trillions of viruses carrying the recipe for the base editor into her spinal fluid, she died of a severe immune reaction linked to the therapy, Science and Retraction Watch can now reveal.

In a possible future, itself perhaps locked in to some other timeline, your dreams may be possible for your grandchildren and great-grandchildren. But today, they're just as likely to get you dead.

when you can digest cellulose but you lose 90% of your daily fiber and get colon cancer

Yes. after it reaches a certain point of maturity. Gene editing hasn't.