I think this would be too easy. Just through normal gameplay, I, and some of my players, have gotten 7-8 startbases before. If everyone can build 4 you would have to make the win 10-12. And even then it would be easily countered if the players just don't build 1 starbase each.
But you can just hold on to a Starbase that an oposing player has built to prevent them from building a 4th Starbase as they would have to have all 3 of their original Starbases to build the 4th one. To be honest I find it hard to win the game without at least 1 Starbase.
As for holding 5 systems vs holding 3 home systems I think players will defend their home systems stronger than Starbase systems. But yes this is to get those players from holding only their home systems to holding key strategic systems as well. The 5 starbase system is a way of forcing some interactions between the players as a player can only build up to 3 (4 if you use the 4th starbase optional rule). If the players know what they are doing they will know that they may need a place to build ships from that is not their homeworld. So getting 5 Starbases should not be impossible.