Since I keep having problems with DoH (Nextdns), can I enter a DNS server (e.g., 1.1.1.1) as a fallback, or am I misreading the manual?
Unfortunately no, regular dns are used to resolve doh domain, in case DoH is failing you can only have a script.
Redundancy is possibile only using regular DNS.
You can use this trick to make RouterOS use multiple DoH servers from different providers simultaneously:
If one of the DoH services fails, you'll still see some error messages in the log, and a few queries might fail, but afterwards they will succeed when RouterOS switches to the other servers.
But of course, this config will distribute your DNS queries over the listed DoH services, and you cannot prioritise just one and use the rest as failover 
ok, sounds nice ! … what about the server settings in ip/dns ? has to be removed ?
It's not important. You can keep them if you want. Because of the regex rule, the domain lookup will not use them anyway.
@CGGXANNX Do I need the IPv6 entries even though IPv6 is disabled under IP/IPv6?
Do you mean the static DNS entries to bootstrap the DoH servers? You don't need them if you don't use IPv6, but leaving them there is also not a problem, because they will simply not be used by the resolver.
Very clever... what do you use as use-doh-server= ?
Because the static FWD record with the regex already catches all internet domains, what you put under /ip dns use-doh-server is not important. But you should still put an URL of your choice there, so that you can disable the FWD record at any time and still have working DoH (by falling back to the main setting).