Cant you take the battery with you and lock it up so that to get your bike they have to cut through the frame.
My weird mind just came up with a perfect solution...
Your e-bike battery probably contains 40 or 50 separate 18650 cells, right?
Imagine you simply wired these end to end in a flexible/protective tube, so you have a battery pack resembling something like a long string of sausages/chipolatas. Obviously, there’d need to be some wiring running along the inside of this flexible/protective skin to “group” the cells properly too. With me so far?
In use to power the bike, you coil up this battery chain in a spring fashion (granted, it would need some imaginative mounting bracket), and it would provide the normal battery power for the bike.
And when you park your bike, you simply uncoil this battery string and use it to tie your bike like a chain to the nearest lamp-post. Both ends “connect” end-to-end using a small, pocket sized, lightweight locking “connecting box” with an alarm inside. It doesn’t need great strength to withstand forcing... it’s the continuity of the battery circuit that prevents the alarm sounding. This “connector” could use a numbered touchpad passcode, or a phone app to arm/lock (it’s powered by the battery, remember).
Here’s the brilliance...
Any movement/tampering/connector fiddling or if the battery string (continuity) is broken, a watch-sized reserve cell in the connecting bracket sounds the alarm (as car alarms also work). An alert could also be sent to your phone. Breaking/damaging the battery string also means that the battery isn’t going to then power the bike to aid the thief’s escape either! To be honest, for as long as the battery string isn’t physically broken, you could use this much power to electrify the frame... and simply spark/fry the git alive... much like a Taser.
On the other hand, if the battery string is forcibly cut, the thieving git gets INSTANTLY flamed by cutting into a lithium cell/the wiring thereby also shorting the circuit. Instant revenge! And, incidentally, continuity would also break, sounding the alarm... and attracting the attention of passers-by who could watch the thieving git getting flamed. Of course, it’s likely that by time the flames go out and an ambulance is called, your bikes’ paintwork might be a bit singed.
BUT you don’t need to carry either the weight of a normal/decent lock on your bike to your destination, NOR do you need to carry the weight of your battery with you when you leave it!
Well, I thought it was good. Maybe I’m just too angry about thieves generally.