Lock washers for problem areas

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Lock washers would work, I also suggest a little thread locker like super glue on the foot platform screws.

I trained and worked as an aircraft mechanic years ago. These aircraft used standard hardware. The Streetstider is metric.

One of the few places I recall lock washers was under the head of the piston engine valve cover screws.

Elastic stop nuts would be a better choice or when the bolt mostly has a shear load and rotation is allowed, a drilled shank bolt with a castle nut and cotter pin or a pin that looks like a safety pin.

With elastic stop nuts, at least 3 threads should be visible past the elastic material to ensure the elastic is fully stretched by the threaded portion of the bolt. Too many threads showing could mean the nut was stripped on the unthreaded portion of the shank. The solution would be a shorter bolt or washers used to shim the nut so only 3 to 6 threads show.

It used to require degreasing before applying threadlock (e.g. LockTite). Grease and oil reduced effectiveness. I don't recall it being used for structure or control surfaces.