There are other considerations as well. France charges access partly by the number of axles. All French trains of recent manufacture (Reseau, Duplex, Regional Bombardier) have shared bogies. ICE trains have four axles per coach, as do the new Eurostar train sets. I've always understood the shared bogie set up to be safer in a high speed derailment, not they these happen too often, thank goodness.
As MoatLane has pointed out, you don't get discounts on Eurostar, so I rather expect our preferred cross Europe journey will be via Lille, that is if the fares are cheaper than continuing all the way to Paris. Then an overnight stay in Lyon, perhaps, a far better option than Paris (why? cheaper hotels, better food, friendlier people). We are retired, after all, there's no hurry and this is not a race, though that was in part the origin of this thread! Paul Theroux put it well, the journey can be more exciting than the destination.