You need 4 Traveldays of a Flexipass
I assume the system counts the time you are actually on this trip and this would be 6days from 19th April to 25th April :)

A 4days in a month pass would be enough You need a reservation for the Eurostar (check before if the train you intend still offer seats as the seats in Eurostar for Eurail/Interrail are limited)

From Brussels to Frankfurt use the ICE and avoid the expensive Thalys :)

 

Thank you so much Thalys!

That seems like a pretty big flaw for a journey planner that suggests the ticket option you should go with, but glad you could confirm I’m not being mad.

Do you know how I can for the Eurostar seats? I’m happy to reserve from now, do I need to just try and book it and see what comes up, or is there somewhere I can check which trains still have interail seats?

Caroline

Iam not Thalys Thalys are the expensive trains between Paris - Brussels - Amsterdam/Cologne :D

Yes you can search via -Beurope (but to reserve you need a PassCoverNumber if you book a MobilePass you have to ask Eurail/Interrail to provide you with a PassCoverNumber)
You can reserve even directly via Interrail/Eurail

https://www.b-europe.com/EN/Booking/Pass#TravelWish here the link to check for free seats  

Super helpful, thank you!