You CAN maintain the friendships once kids are in school. My DD is 6 and I had the same concerns. We've tried the homeschool circuit but haven't really found a good match in the area yet.
I work hard to make sure we keep in touch with the friends we didn't want to lose once everyone went off to preschool then elementary school.
I just got an email today from one of our friends whose daughter finishes school this week. We see them every school holiday, the girls go to each others Bday parties and see each other at mutual friend's parties. We celebrated NYE at their house. She was emailing me to get together next week for a picnic at a local farm.
This past weekend we went to two different school carnivals with our friends who attend those schools. We do Girl Scouts at one of the schools so we knew many of the families at the carnival.
I organize a Mom's Night Out each month with a small group of women that I want to continue friendships with. They all have a child within a year or so of my daughter and we've known each other since they were all under two years old.
We've started doing a morning out as well every couple of months, everyone comes with or without kids depending on school schedules.
I offer babysitting for a couple of kids my daughter really likes. It helps the other parents out and it help keep the girl's friendships strong in between playdates.
We stay active at church where DD has a few friends her age.
DD isn't seeing her friends every day like school kids do, but she does have a good size group of friends and we see several each week.
Another thing that helped us was to remain in our Meetup playgroup even though the other kids DDs age were in school. Many of the younger kids in the group have older siblings who come to events during school breaks. We don't do weekly playdates with the group like we used to, but we go to the holiday parties and bigger events.
Even without a homeschool group we find more than enough to do each week. Library programs, community programs, parks, church events, scouts, gymnastics, music class, etc, etc, etc.....
We were lucky to have a couple kids with half day kindergarten too, which bought us an extra year or more flexibility with those friends.
It can be done. It takes work, but it's nice to be able to pick and choose who you and your kids are spending time with! Your friends with kids in school don't have that luxury.
It's hard not to worry, but trust that you are making the best choice for your family!