You're too early for Italy I believe.
It is always best to avoid eurail.com for seats when possible (they add 2€ fee + sometimes mail paper reservations while they're available online)
General advice :
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
https://www.seat61.com/interrail-and-eurail-reservations.htm
Specific advice :
- Brasov - Budapest tickets.oebb.at for the 16:21 train. Someone else will be able to help for other trains.
- Vienna - Berlin 3€ per train on tickets.oebb.at or 4.50€ per journey on bahn.de (optional but recommended). For the night train (book ASAP) tickets.oebb.at or nightjet.com (add Eurail as a discount)
- Amsterdam - Paris 32€ on eurail.com if you're alone or https://www.b-europe.com/EN/Booking/Pass#TravelWish (4€ booking fee per order)
- Paris - Milan 32€ direct TGV on eurail.com (could save money going via Switzerland)
- Milan - Venice 10€ on tickets.oebb.at