When are you planning to travel ? Thisis the defining question. Maybe well in advance tickets would be cheaper but a pass will probably be best. :)
Keep in mind the mandatory seat reservations on top of the pass in France and Spain :
- TGV : limited 10€ fare, then 20€
- TGV from Belgium : 20€ with a passholder quota (often better to pay for a regular TGV ticket to Lille or to travel via Tournai and board it in Lille)
- Thalys to Paris : 27€
- trains in Spain : 6-10€
Have a look at these 2 links :
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
https://www.seat61.com/interrail-and-eurail-reservations.htm
Do not hesitate for further questions. Je parle aussi français si jamais.