I did a quick route plan with ABRP, Calais to Marseille, 1st using Ionity 2nd excluding Ionity. 1 hour 20 minutes longer not using Ionity. Personally I'd plan to use them and swallow the cost or just not bother. If you are going with family you really don't want to turn this into the "do you remember when Dad made us drive down to France in the Niro" family story , let alone the aggro at the time.
Your going to have to (as you are) do a lot of research here, I'd check out electric felix's blog as the info does update, certainly check plugs surfing / charge map / shell / total websites to see what chargers on your route outside of Ionity you could use with what cards, befriend someone with a first edition we charge card and get some free charging (?) ..
Without wanting to sound like a smart ar$e this is why we bough the Tesla, every year we travel down to the sister in laws in Italy at least once, quite often twice. I wanted to do it in an EV and Tesla are the only people who at the time made that route (Genoa) and many more possible without having to worry endlessly. Compared to ICE it's actually easier as we've had many a problem in Italy with unmanned petrol stations not taking UK credit/debit cards. It's very frustrating that the other manufacturers haven't got their act together on this and even worse with Ionity that with a patchy network half built they then turn the screw on price