If we get 2 WAR worse at 2B and get 2 WAR better at SP I don't think it makes a difference at all. A SP in the playoffs only throws 4 IP now apparently anyways so their value is even less in the playoffs. Our SP just happened to be performing terrible this round, but Boyd and Shota aren't bad pitchers, they're above average but struggling at this point in the season. We do need to acquire another good SP though.
You could resign Nico to a 4 or 5 year deal or whatever. He's also a SS for teams so we need to keep that in mind. He could get 5/125 or something like that. But we can't expect him to remain a 4 WAR player in his 30's. From age 30-35 I could see something like 4.0 WAR, 3.5, 3.5, 3.0, 2.5, 2.0. Would I pay 5/125 for that? No.
Shaw can play 2B also so we have to find either a 3B or a 2B to replace Nico and finding a 4.5 WAR guy there probably won't happen.
Resigning Tucker probably nabs us around 2.0 extra WAR over whatever his replacement would be over the next 4 seasons or so until he starts to regress. If the Cubs can figure out a better way to spend 350 million or whatever over the next 8-10 years that nabs us more WAR then do it. e.g. 1 WAR upgrade at SP and another 1 WAR at another position does that. They could literally just trade Taillon and replace him by signing an above average FA SP (I consider Taillon an average SP) then sign another above average SP. Boom 2 WAR.