Thanks for the input.
At this point, the longest day I did on my six-speed Brompton was 150km, but I was totally exhausted. OTHO, I can ride 100km several days in a row just fine.
Considering the Brompton is closer to a regular bike in terms of body position (distance of handlebar to saddle), I doubt I’d be able to do 100km days with a three-speed Strida… but I might just try for science’s sake 