I like to bike when I can, but I’m still nervous to leave it even with a U-lock around the frame. I’ve also had a seat stolen before (not even a good one), but I’m not sure if there’s anything reasonable to do about that.
What do you do to lock up your bike for commuting?

I use a thick chain on my ebike. I forget if it’s 11mm or 13mm. But it is thick AF. I also have a Ulock for extra protection. Your city might also use a locker program, which might be administered through something like Bikelink. If so, use it. The lockers are usually spread out in high foot traffic areas like transit hubs.
If weight is a concern, I’d get a ulock like the Hiplok D1000. It’s going to take thieves about 10 cutting wheels to get through.