Right—some effects of taking more systems! In general this moves the future peripheries forward, but this is a specific list of exactly what happens. Doing so sooner is cheaper than doing so later, and the biggest advantage lies in reaching attackers which we would have had to clear separately anyway. Note also that there is no major strategic need to keep Spire sites; the final system has one, albeit not the most popular, yet not the worst either. Strengths involved are at around 6000 and rising.
Relative to the present plan, taking one more system this week 74 exposes an attacker next week 75 which I had planned to clear anyway, giving a strength saving of around 700. Nothing else changes immediately (because it was planned anyway), although the extra future time could then be used to make other simulations and adjustments.
Taking two more systems would destroy the popular Spire site in week 77, but also would prevent it from attacking, such that week 78 begins with 8 Control plus an Alert and therefore week 79 begins with resistance 0 rather than resistance 1.
Taking three more just spends time now to save time later in week 78.
Taking four more would stop that other Alert in week 77, such that week 78 begins with 6 Control only. Definitely that will attract Titan attacks, although probably we should still clear it away.
Taking five more would reach a planned attacker clearance in week 76. We could then change target to stop a second Alert, causing week 78 to begin with 4 Control only. That seems not particularly desirable; it is a lot of work and a higher-resistance Titan in return for exploding it one week earlier.
Taking six more would be the same as five, but with spare time in week 76 to clear arbitrary further systems just to get a lower Titan resistance in week 78. Taking seven or eight more would free time similarly in week 75, again just for accelerating the process for a Titan in week 78 rather than week 79. At this point we are very much just clearing a double-periphery.
Part of the reason for stating specific weeks in which an effect occurs is to note that the advancement amount can be done across any weeks prior to it; that is to say, the only effects which require all of their extra clearance amount now are the ones which occur in week 75.
In any case, having listed it all now, I presume that aiming for two more to get a zero resistance Titan will be a popular choice. By all means express preferences though!