Assume all the Hightower dragons are girls, how would you rate Rhaenyra taking the throne with Daemon and Jace and Baela after her? No dead dragons, solid and loving royal family. Would a war still follow?

There’s absolutely going to be a war, regardless of gender. Bottom line, if Viserys didn’t want a civil war, he shouldn’t have formed a competing dynastic alliance with House Hightower. For Westerosi highborn, marriage isn’t about love, it’s about property and political power (which are the same thing in feudal societies).  House Hightower handed over their daughter, a valuable political bargaining chip, because they expected political power in return, and Viserys knew that even if he didn’t want to admit it.