Somehow answering my own question (though not in a satisfying way, as after all the hypothetical device I was looking for seemingly does not exist).
It seems like Mikrotik routerOS (probably also other routers/OS's but I didn't find any evidence about that) through some "magic" tinkering with prerouting/mangle/connection marks:
allows addressing from a same router more than one device with the same IP address.
So with two routers in cascade it would be possible (maybe) to use the first as the failover engine (recursive) routing from itself (192.168.1.1) to three different IP addresses (on a different subnet, let's say 172.16.0.0) that are managed by the second router that "routes" 172.16.0.10, 172.16.0.20, 172.16.0.30 to the 192.168.1.1 on ports ether2, ether3 and ether4.
If this works, the two routers can be (in case any of the two fails) by-passed by manually reconnecting cables (to only one of the DSL/4G routers, as it is now) without changing any configuration on the network clients.
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Somehow answering my own question (though not in a satisfying way, as after all the hypothetical device I was looking for seemingly does not exist).
It seems like Mikrotik routerOS (probably also other routers/OS's but I didn't find any evidence about that) through some "magic" tinkering with prerouting/mangle/connection marks:
https://forum.mikrotik.com/viewtopic.php?t=187178
allows addressing from a same router more than one device with the same IP address.
So with two routers in cascade it would be possible (maybe) to use the first as the failover engine (recursive) routing from itself (192.168.1.1) to three different IP addresses (on a different subnet, let's say 172.16.0.0) that are managed by the second router that "routes" 172.16.0.10, 172.16.0.20, 172.16.0.30 to the 192.168.1.1 on ports ether2, ether3 and ether4.
If this works, the two routers can be (in case any of the two fails) by-passed by manually reconnecting cables (to only one of the DSL/4G routers, as it is now) without changing any configuration on the network clients.