It sounds like the issue is less about managing the team and more about managing expectations. As the OP stated, when you don’t have people there you don’t get much done.
So, how to get people there?
While every team is different, I personally think it’s important to recognize what the program is all about - it’s not about winning a robotics competition, it’s about inspiration (with the competition being the means we use to help us inspire). For that reason, we don’t have required hours. We don’t kick people off the team. If they show up once a week, we do what we can to inspire them and help them understand what STEM and Engineering is all about.
We do, however, provide incentives for showing up. Students need 50% attendance to travel with the team, based on the originally published schedule (deviations from the schedule due to snow days or last minute work are generally held in the students favor - if the team cancells a meeting, the denominator goes down, and if there are extra meetings they can be used as extra credit). Further, students have to achieve 80% attendance to letter, among other requirements. Those incentives alone are enough that we’ve only ever kept a couple of students from travelling with the team, and almost all students letter after their second year (not eligible their first year).
One thing that is important is the timing of your meetings. Many (most?) activities happen right after school and run until dinner time. We meet later in the evening, from 7-9 (necessary so the mentors can get there after work and a fast dinner). This drastically reduces the conflicts we have with other activities during the week. Some students will come to the robotics meeting straight from practice for another sport, cramming down a packed dinner as we get started. Other students who don’t have another activity will hang out in the school working on homework, and arrange dinners as needed. Some, of course, go home after school and come back later for the meeting.
Finally, when you have a schedule set at the beginning of the season, ask the kids to indicate approximate expected attendance. Hopefully, they’ll know what activities they have that will conflict, and can provide you with the appropriate expectations. Then when your planning strategy and robot Design, keep those estimates in mind to pull th hem back from an “everything” not to one that you can accomplish with your resources and time. After a year or two of telling them " we don’t have enough person-hours to do as much as you want", they’ll get the message and help change the culture of the team from within to get more dedication and commitment.