This is pretty off-topic for the original topic, but I wanted to share with you our joint custody arrangement over a distance (in our case a much bigger distance, but somewhat applicable).
We (the ones who moved) share equally the cost of transportation for school breaks (the parent pays for the transportation required to start their parenting time... so we pay the cost to get her from mom and mom pays the cost to get her from us). We pay 100% of the transportation for any other visit (and the year mom wanted a mid-summer visit, she paid the full cost of that travel). We are not within driving distance by any stretch of the imagination, but besides school breaks we have one weekend a month (with priority for 3-day-weekends if one is available) plus an additional 2 one-week periods during the school year (exercised in mom's state because she has to still attend school). We have to give 30 days notice for which weekend, 45 days notice for the two weeks, and mom has to give 45 days notice for any weekends she wants to claim for her own (so, for example, if they have a big family event and she doesn't want my husband to exercise his parenting time that weekend). We have to be flexible because we are buying plane tickets, but if we were driving it might work better to set a specific weekend (mom isn't always super thrilled she can only plan her weekends 30 days in advance). Mom is (theoretically) required to be flexible regarding additional visits, but the reality is that, when we are already spending $400-$1000/month in transportation, additional visits aren't really practical.
Personally, I would propose something similar-- one weekend a month at mom's, and additional visits in the kids' hometown if mom chooses, with reasonable notice. That means the kids are only having to make the trip once a month, but they can see mom more frequently if she were willing to put in the effort. Personally, since she is moving, I would ask her to handle the transportation on her weekends, but that's probably where I'd be willing to compromise and offer to meet halfway once a month. Then for school vacations, I would share the cost and driving.
Also, we were granted EVERY school break, in its entirety, but we think it is important for my step-daughter to spend some holiday and vacation time with her mom's family. So we give up half of Christmas break every other year and 1-2 weeks in the summer so she can have that time. You might want to consider if that is something you would want to have as well... while you certainly want to maximize the time they get with mom, I would caution against you guys NEVER having any vacation or holiday time with the kids, especially as your family starts to expand and you want all the kids to be part of those holiday and vacation memories.
For us, child support is calculated based on the state calculator, regardless of what one parent or the other wants or feels like. In our state, child support is calculated separately from custody. If it were me, I would make a custody agreement that doesn't mention child support (mom will think she is getting out of it because you aren't mentioning it), then file for child support after you have a custody agreement. I'm not saying this to promote dishonesty or some sort of trickery, but because the child support discussion should be separate from the negotiations about visitation (so neither person should be able to use it as leverage), and if you file for it through the child support agency it will be calculated based on the factors your state considers appropriate, not what one parent or the other decides they feel like paying or receiving, and they are based on actual documented income, not a guess or a fictional disclosed amount.