There’s a guy on youtube (The Thought Emporium) that did something similar so he could defeat lactose intolerance, however temporarily. For a while, until his gut shed its lining, he could synthesize lactase again. So doing the same with cellulase isn’t that far fetched. What I would really be worried about is the prodigious amount of gas you would generate, as our guts aren’t really mechanically built for that degree of rear-gut fermentation. I think you’d have to start with an elimination diet of some kind (keto?) and work forward to small amounts of vegetation to test what your body can handle. It’s all doable, but risky.

https://www.youtube.com/watch?v=J3FcbFqSoQY