I have two Juniper routers (a SRX340 and a SRX320), each with a leased line, on the same IP range. They are designed to be connected together on specific network ports so that the SRX320 takes over if the SRX340 fails.

I also have two SonicWALL TZ600 firewalls, currently configured in "Active / Standby" high availability mode.

I want to connect the routers to the firewalls in such a way that any one device can fail without the whole system losing internet access. What options do I have, please?

EDIT:
I have this working now. I connected the LAN ports from the two routers into a single switch, and then from there into the X1 port on the two firewalls. Each device can be switched off and internet access is maintained (after a minute or so of downtime). All I need to know now is how to eliminate the single switch as a potential single point of failure.