I just wanted to follow up about this trip, before I forget (having flown home just yesterday).

Overall excellent, and I really enjoyed biking in Germany and Austria --- I hope to bike more in Europe. Overall great and well-marked dedicated bike path or low-traffic roads, and even in the cities it's just great --- so very many dedicated bike lanes, you can really get around even in big cities like (most recently for me) Vienna. Overall the opencyclemaps project does a good job, too, of laying out good biking options.

Both Lake Constance and the Danube bike path (Donauradweg) were great experiences; I reckon we biked 700-some miles in total, keeping daily mileages generally low.

As to using folding bikes for this, a mixed result but overall I don't think I would recommend it. Even on a relatively flat path, it IS more effort pushing along a bike with 20" diameter wheels, and the folding feature didn't help us too often.

It DID help on occasion, however. At one hotel we didn't like the option of parking them on the street, so we folded them up and brought them into the room with us. Our last train ride lacked the bike car we had bought bike tickets for, so we had an easier time of making do with folding bikes there. But in practice we found it easier to buy bike tickets on the trains, so that we could just keep our panniers packed and roll the bikes on and off, and almost every place we stayed along this well-known bike trail had dedicated bike parking for customers, most often indoors or at least covered.

The low-end bikes we bought did fine overall. We both had a flat, one each, both times discovered in the morning; but we didn't have terribly expensive tires. I had to replace on of my tires, my wife just had to patch her tube.
The folding peddles on my bike went wonky, but fortunately in Passau we were staying not far from a really really big bike store, so no problem buying better replacements. I still like the folks who sold us our bikes (Origami), and to be fair I think we got good value for our money, and would suggest that bikes at this price point aren't really meant for longer distance touring (!), but more for casual use. Ours survived the trip to and from Europe just fine in the plastic suitcases we bought (also from Origami) for the purpose. We found the process of disassembly and reassembly (to fit into the suitcases) was more time, effort, and greasy hands than we really wanted to deal with, however. If that Canadian Helix project ever comes to the point where one can buy the bikes, that sure sounds interesting --- amazing if they could get a 24" wheeled folder into a "not oversized" suitcase without significant disassembly. If they really can, I'd be interested.

One thing I noticed in Germany was how very many eBikes there are, electric bikes. Very very common, and as we have aging demographics too I could see these getting more popular in the U.S. On occasion as older, and in my mind at least, less fit folks would pass me going uphill (using their electric bikes), I couldn't help feeling a bit jealous.

I like the idea of an eBike if it means I can go farther or on tougher (hillier) terrain than I otherwise might care to and have fun while still getting exercise. And you can buy folding eBikes. The catch is that they're a lot heavier, and that same aging demographic is going to have trouble carrying a 50+ pound folded bike onto a bus or lifting it up to put into a car trunk. The ones with removable batteries can mitigate that problem some, I suppose.

Bottom line is that I loved biking in Europe (though it's certainly not an inexpensive vacation), and would consider a folding bike again for such a trip if that Helix or similar folding bike advance were to offer a bigger wheel diameter and no significant disassembly to take it on an airplane.