Fed up with my broken Strida (Toronto CANADA)

My best regards to your mechanic, but I don’t think so.
First should the plastic freewheel be changed, I mean it’s clearly visible that yours it totally worn out.
And that will be the reason why the belt seems too long now.
(The active diameter of the plastic wheel is much smaller.)

Posted here:
[url]Strida 5.0 upgrade DIY]

you can find pics of a worn out freewheel, these:

and here are pics of a, maybe not brand-new, but much better one:

Do you see the difference between the teeth?
Maybe you should show above pics to your mechanic?