There are a number of things that could be done to adjust it but there are a lot of factors to consider
I think the biggest factor is that Tri-color factors into the (Open) battle results during the final Splatfest Tally. There are 35 points total during the final tally; Popularity, Open battles, and Pro battles. Of those 35 points, 15 go to Open and that is an absurd amount. The only scenario that a team that wins the Open category is for one team to win both popularity and Pro battle categories. That could have worked when there were only two teams but now there are three with their own votes and wins dividing the player pool
The first thought of what needs to be fixed is we need a fourth category during the final Splatfest tally. It should be Popularity (10), Open (15), Pro (10), Tri-color (10) for a total of 45 points to be distributed across the 3 teams. This way it becomes more a factor of which team wins two categories with the 15 points acting as the tie-breaker rather than one team winning one category to gain a massive lead.
Next is how players opt into battles. Right now the leading team can only choose Open or Pro settings when playing, and get forced into Tri-color battles at random if there are enough suitable players, while the two Attacker teams can choose Open, Pro, and Tri-color. I think that should remain the same for the lead team. They pick open and if they join a Tri-color battle, the results play into the Tri-color category in the final tally, but if there are not enough players or matchmaking takes too long it becomes another 4v4 battle that counts toward the Open category allowing the leading team to defend and extend their existing lead. Attackers should also have to choose Open and be randomly placed into Tri-color battles to factor into that category and if they can't start a Tri-color they get put into an Open battle.
Lastly I was thinking that the top two defending team members should get a Festival Shell that applies to Open and Pro battles if they lose a Tri-color. More Festival Shells improve the chances of a 10x or 100x battle starting allowing the Defenders the chance to push their existing lead in Open and Pro categories to cushion themselves in the likely defeat they face from the Tri-color category in the final tally. Making the Tri-color worth its own points would also promote the two Attacker teams to fight among themselves during Tri-color instead of treating it like they both won whenever the defending team loses.
I think issues with the final tally distribution need to be addressed before any specific rebalancing to the mode itself, which any change could wildly swing the balance in the opposite direction. In the full game we'll have more specials, weapons, and gear abilities to work with so that alone could cause massive changes to how the mode plays out. But what we have to accept and be mindful of before considering any type of balance change is the fact that Tri-color exists with the intention of giving the losing teams a chance to comeback by forcing the defenders to be at a disadvantage. We can't take that away so the objective is to make the mode not be such a blow out for defenders, which is why I think adding a new category to the tally will give a suitable avenue for the attackers to aim for without doing excessive harm to the leader's existing lead. After that there is all manner of options to go with. Reduce the amount of paintable turf in the Attackers' respective spawns, extend how long it takes for the Signal to activate when it gets grabbed, reduce the amount of ink spread of the sprinkler, extend the respawn time of someone that dies while attempting to use the signal, so on. The thing I was thinking of is it should be easier for the defenders to go into the attackers' spawns like with defender color sponges but what I would add is an ink rail from the defender's spawns that lead them to the far end of the attackers' lower level of their spawn. It takes longer to get there but now the defenders can paint the home spawn of whatever team is winning / got a Sprinkler. Like the rail could be one of the automatic ones from Hero Mode and Salmon run so players can't camp on it and they could activate after a signal successfully activated, with the first rail leading to the team spawn that activated the Signal. Maybe even have both rails lead to the same spawn from different angles if the same team gets both Signals. It gives the Defenders the ability to put more pressure on the team with the sprinklers and with the defenders so focused on that one team allows the third team more freedom to try pushing in themselves and sneak a win on their own