How much will we realistically have to book in advance?

That entirely depends on where you're going and on which days you'll travel. Fridays and Sundays are always busy and in some countries it is easier to find (partly) reservation-free routes.

It really all depends on where and when. In fact the big majority of all EUR trains do not need any REServ nor often even possible. But most newbees all want to ride the very same oversubscribed trains with many problems.

F.e. in FRance the major 14/7 holiday leads to massive movements-and again the start and finish of their ´official´ hols on 1/8 and then 31/8. But in Scandinavia the hols start mostly on 21/6=midnightsun.

More or less same applies to hoStelbeds-except that due to covid it seems that in major toruisty towns many hoStels of old time have closed for now. But with 2 a normal hotelroom is often even cheaper as 2 beds-but then you dont have the extra freebies like kitchen, social room etc.