Very. It's a lot more complicated than just a battery, it's a complex management system that, despite the reputation of Tesla for manufacturing details and their CEO for...well, himself, is very impressive. You'd need to keep or reinvent most of that to avoid cell failures (and fire).
If you're really serious about a small DIY EV, look into the kits that are available to modify existing ICEs into EVs. Not quite the power level, but they do work pretty well if you don't mind the complexity of the work in strip down and rebuild. And often times they do use used/refurbished Tesla batteries in their makeup.
Understanding the variety of speech over a drive-thru speaker can be difficult for a human with experience in the job. I can't see the current level of voice recognition matching it, especially if it's using LLMs for processing of what it managed to detect. If I'm placing a food order I don't need a LLM hallucination to try and fill in blanks of what it didn't convert correctly to tokens or wasn't trained on.