The Flag Curse upgrade is a great addition and can help alot, but with the new instant tnt nerf it can still be hard to return the flag when the entire team is protecting it. My suggestion is to make the curse affect nearby players of the same team and reduce them to maybe 6 hearts. This would help alot.
Also the curse vanishes as soon as the flag is returned and people can take the flag as soon as it is returned. Make it so that the curse vanishes when the flag is returned for 10 seconds, or when a flag is captured.