Noone will ever be able to tell you if a train is fully occupied or not and from when-PLUS that for overnight there are various options, like seat only, sleeper etc. The good news is there is more as just 1 train-via various routings, but some are quite old/vandalised and not overly comfortable.
Are you really sure you want to use a pass for this? Normal tickets are now available with some patience on cfr.ro, and with some luck you may even score a special price (valid only that day/train)-which may out cheaper as passday+fee for REerv. But seat-only REServ are not possible that way.

Seems a bit strange to me to land at Buc and go immediate to Bp-fly direct should be much more convenient. But yes, I know a lot about silly airport lines, prices, bookings etc.

Since a few monthes and probably for some time to come, many refugees from Ukrayna use these trains (for free) to reach shelter in the west. Due to the war from the Russky.