I had the exact same problem a few years ago where all worked 100% until new iPhone and iPhone update. I had to delink the watch (Bluetooth) from all Apple devices, including old one, as it apparently could not discriminate between them and the various IoS versions. Here I mainly used the iPhone but had in the odd case used an iPad and Mac. I just had to bash my head as Fitbit support was useless.
I recently bought a Charge 3 and it synced first time and is running happily on same phone.
Unfortunately, as the watches are not charged it is not possible to do test this before purchasing. Also most retailers are just middlemen, and most are totally unaware of such issues and get very defensive even though they are ignorant. They just want the sale and commission.
Oh and I must add, that when I got my new Charge 3 I had the exact same issue with messages but after a day or two they started coming through and all now works 100%. Had the same with calender etc. It appears it takes some time to synch to all facets / apps on the iPhone.
