If you get a flexi pass (X travel days in 2 months) you have to quite slick in your travel plans, especially on overnight journeys.
A continuous pass is totally flexible and allows you the “luxury” of short and long journeys without worry or having to buy tickets for regional journeys, i.e. those not worth a Flexi Travel day.
Remembering that you are likely to be doing a lot of long distance travel I would recommend you also cosider first class. In the full scale of your budget the cash difference is small and the luxury of wider seats and some at seat service and lounge use. Also if luggage is your concern there is normally much more space in first class, not least because there are quite a lot fewer passengers in the carriage.
There is nothing wrong with travelling second class but first is better.