Does the local transportation company provide an app for iOS itself? Maybe that could be an option for you to not use google wallet.
I have chosen a local transportation company to not use the DB navigator app which is a privacy nightmare and support my local transportation company financial.
I use immich and nextcloud for the clients (my wife and my parents know that I only take care about that data) and on the server side I use borgmatic which has a local repository on the second drive inside my nuc and a remote repository hosted by hetzner called "storage box" which supports borg native.
Yes the remote is out of my physical access, but borg is fully encrypted and for 4$/3.6€/month for 1TB I feel good.
Before I started with borg and hetzner I had a rsync based backup with an odroid hc1 hosted by my parents, but that doesn't feel safe. Due to slow network by my parents I had to sync my local backup instead of a second backup from the real data and the monitoring was also very bad.
From my point of view: You have no backup, if it is not automated and you have no monitoring.
Does your router indicate that you have DS-Light? I think O2 provides each customer DS-Light until they ask for a real IPv4.
To your second question: In case of DS-Light you don't need a new IPv4 IP every 24h because your IP is not public facing.
PS: I don't be sure, but the Fritz Remote Apps use IPv6 to ensure that they also work with DS-Light.
I host synapse as docker container behind traefik and it works pretty well. I have two users on my instance, have setup the mautrix-whatsapp bridge and federate with the instance of a friend.
The setup was straight forward: Pointing the sub-domain via traefik to the service and in the homeserver.yml enable well-known which announce port https with port 443 instead of 8448.