Note: That bike came with a single chainring, that I needed to replace with a dual crankset to get a wider range for touring. Hence my problem.

Folders can be quite different from standard bikes.

In this particular case, even with the original 127mm, BSA 68mm Kenli KL-08A BB it came with, the fork of a dual derailleur can't totally move inboard (it hits the rear triangle) enough without the chain slighly rubbing againt the derailleur fork; Moving the big ring yet further outboard with a 2mm spacer or a triple derailleur makes it difficult or even impossible to work because of its ~60mm chainline.

I didn't know those "E-type" derailleurs. Provided I do find one second-hand since they're no long manufactured, can they handle a ~60mm chainline?
https://www.ebay.com/itm/226344804407
https://www.taylor-wheels.com/front-.../3x8-top-swing