Been taking our son (now 7) on 'road trips' since he was 4 months old. This included a 5 month stint in France, Spain and Portugal. We have a Motorhome. Kiddo LOVES it. We never stay anywhere more than a few nights. The above countries (plus Italy, Germany and Belgium) are all ACE for motorhoming. They are stuffed full of mainly free aires. These are dedicated areas in towns/ villages/ cities for Motorhomes to be parked up and slept in overnight. Most have services - so you can ditch your grey and black waste and fill your fresh tank.
Now my son is at school and I'm back working full time, we are limited to 5 week trips. If I were you I would go for 4. It gives you time to really chill, enjoy and explore. I'm biased but would go for the Motorhome hire option. You have your home on wheels wherever you go. So no faffing with cases, unfamiliar beds etc. Although the hire would be pricey if you compare that to 4 weeks B&B and eating out... it may even be cheaper. We always self cater.
2 is a great age... 2 year olds are so easily pleased! We now tend to be more choosy with where we go to ensure there is 'something' for our son. You really don't need to do that with a 2 year old, give em a stick, point out cows, tractors and planes and they're happy! Therefore I would pick places YOU want to visit which attract you. We are very outdoorsy and into sport so head for mountains, lakes, rivers... Other people might be really into French food, or art or history and holiday accordingly. At 2 we continued to do what we'd always done - cycled, walked and canoed. But just did it with a trailer, sling or tag teamed. You can do that with 2 year olds.
When he was 2 we tended to just drive during his nap time. So that gives you 90 mins drive time daily. When we needed longer drives he would generally quite happily chill in his seat... occasionally I'd pop back to chat/ play with him.
We avoid paid motorway. It is expensive for a start. But the national roads are really good in France. We also see it as being 'about the journey not the destination.' The number of times we've taken the slow route, which is much prettier and more characterful and happened upon places not in guide books. It's about the adventure and exploring. Obviously it's nice to have a rough plan... but again, the number of times we've decided we are going to a set place. Checked the weather the day before and subsequently ended up in a completely different country! You have that flexibility.
Enjoy!! It'll be a fab experience.