Do not keep your settings from 18.06 when you run the update. Upgrading with the settings is not supported.
Make a backup of your settings. Then do your upgrade without keeping settings. Recreate your settings on 21.02 using the backup as a reference if you need, but don't attempt to restore that backup since it will not be compatible.
You could do an intermediate 19.07 upgrade, retaining your settings, and then upgrade again to 21.02, but this is probably still risky. You will be best served by starting fresh and configuring it manually.
I have an unconfigured hardware-backup, a Zyxel Armor Z2 - https://openwrt.org/toh/zyxel/nbg6817 which is very similar to the R7800. Openwrt 19 is installed already, but nothing else.
Can you recommend a strategy to prepare the Zyxel router? My problem is, that I cannot use the static IP-addresses. It would be easy to change IP-address of the Zyxel at the end, but I assume I get chaos, when I define other static IP addresses with the Zyxel.
If i remember correctly the situation with TP-Link, there have been only 2 lines different, when I upgraded from 18 to 19. Should I expect a lot more differences to the 20 upgrade?
Both devices can be upgraded easily, settings can be kept from 21.02.0 (ipq806x hasn't migrated to DSA yet, but will soon, for the next major release), but there will be more significant differences between 19.07.x <--> 21.02.x than between 18.06.x <--> 19.07.x. This means while retaining settings over the upgrade will mostly work, downgrading won't (due to the changes in the netifd configuration syntax).
Given that the nbg6817 is a dual-firmware device (leaving your 19.07.x installation and its configurations untouched on the other partition set), I would suggest not to retain settings though.
I’m not sure what you are asking in terms of preparing/configuring the backup device and what role it plays in this situation.
As far as the upgrade from 18 or 19 to 21 - there are quite a lot of changes. And the documentation says that the direct upgrade is not supported - likely because the methods used to check and update syntax may not work properly with config files earlier than 19.07
There are running routers, which should work without interruption. I want to setup the Zyxel, disconnect the Netgear und connect the Zyxel. This means IP-addresses have to be unique and I cannot configure the Zyxel with IP-addresses, which the Netgear uses. Probably the best way is to setup the Zyxel with a command line editor and as long I don't reboot the Zyxel changes are not active.
You will necessarily have downtime - at least a blip. You can swap in the backup and take the main one offline while you update it, but unless you have a more sophisticated “high availability” network and multiple ip addresses from your isp, you don’t have an option here.
You can configure your backup router with exactly the same ip addresses as the main router as long as only one of them will be physically connected at a time. To do this, perform your config of the backup device entirely offline - the only connection should be between your backup router and the computer you are using to configure it (and power of course). Make sure your computer only has that one network connection (turn off WiFi and unplug any other network connections)
I want to setup wireguard-vpn. Should there be a difference between 19.x and 20.x? I don't find it anymore at ovpn, but I think I read, that the setup is different, depending on the openwrt version
A few minutes down are ok, but I think I cannot test it, if it works as it should. So if there are problems I have to switch back from the Zyxel to the other working Netgear router.
In most cases, it should work. If you are using a cable modem, you will probably have to reboot it (sometimes a few times) before it will connect to another router. You could clone the MAC address of the current router (wan) if you want, but usually a simple reboot of the modem is sufficient.