Legal regulations for eBikes

Hey everyone!

Been thinking a lot about DIY eBike builds lately, specifically how far we can push things legally. I'm planning a custom cargo eBike project, maybe even a mid-drive conversion on an old mountain bike, and I'm wondering about the line between a legal eBike and something that might get flagged as an unregistered moped. What are your experiences or insights on how modifications (like higher wattage motors or throttle additions) impact the legal classification in your area? Any specific regulations I should be aware of for DIY builds?