I could’ve written your post a few months back. My GSD is 10 months old now but I sent him to daycare for the same reasons as you when he was 6 months old.
He hated it. Would sit with the staff all day and just watch the other dogs play and run. He’d be so well behaved there and then come home and go crazy, whereas he was always so well behaved and trained at home.
He’d come home and just wouldn’t be calm, barking for no reason, started resource guarding with older dog and would chase him/play rough. Realised he was just seeing all the behaviour at daycare and mirroring it at home. Stopped sending him after 2 days because it was stressing us and him out. He was back to normal again within another two days.
This was a daycare with 5 star reviews and play equipment etc. Someone pointed out to me that daycares where you can hear the dogs are barking are not a good idea for a lot of dogs. At this place you can hear the dogs barking and see the excitement as they go in and as they come out.
I started him at a much calmer daycare a month or so later. The dogs have to go in calmly and they leave in a calm manner too. They have 25-30 dogs at a time and I have only ever heard a dog bark once. There is no constant barking or anything.
My GSD loves it and plays, is confident and they haven’t at all undone any of the work I’ve put into him. In fact, they correct his behaviour and he gets to play and learn with other doggies. This place doesn’t have 5 star reviews, trampolines or an insta following, but it’s been the best for all of us.