So after hours of spending my time in this shitty configuration, hopefully an expert can figure this out.
According to this thread (german) S2S-Wireguard: AVM zu Mikrotik - Administrator
I created on my Slate 7 (which is connected on Area A to a DSL Modem) a VPN Wireguard config (AllowedIPs = 10.0.0.0/24, 192.168.8.0/24).
(10.0.0.2/24 which gave me my Slate 7, no access to the network, only connection established)
Also I changed the Interface Adress to the Router IP 192.168.178.1 (Fritz Box) on Area B (yeah I know this isn't Wireguard conform, but AVM decided it in their special config.
Deleted both DNS Servers.
[Interface]
Address = 192.168.178.1/24
PrivateKey = blablabla
[Peer]
AllowedIPs = 10.0.0.0/24, 192.168.8.0/24
Endpoint = somethingwhatisyourddns.org
PersistentKeepalive = 25
PublicKey = blablba2222
PresharedKey = blabla333
With this setup, I can import this config to my Fritz Box without any errors.
Also a connection is established.
On my Slate 7 I can also see a green dot behind this VPN connection, so atleast some packets will be accepted.
It doesn't matter on what LAN I am, I can't ping the other network.
Both are DSL Providers with an IPv4 (no shared IP or shit like this).
I think there must be created a special rule in the firewall, but I'm not familiar with that.
Also here is an example when you create a wireguard config on Fritz Box (side 2 side as host)
192.168.200.1 is the host IP and 178.1 the client router.
[Interface]
PrivateKey = rrrrrrrrrrr
Address = 192.168.178.1/24
DNS = 192.168.200.1
[Peer]
PublicKey = rrrrrrrrrr
PresharedKey = rrrrrrrrr
AllowedIPs = 192.168.200.0/24
Endpoint = endpointmyfritz.net:57678
PersistentKeepalive = 25