No, the router being the SPOF (single point of failure) is totally avoidable.
At mny home (no SaaS services offered, but critical "enough" for my life services) i have two different ISPs on two different tecnologies: one is FTTC via copper cable (aka good old ADSL successor) plus a WFA 5G (much faster but with data cap). Those two are connected to one opnSense router (which, indeed, is a SPOF at this time). But you can remove also this SPOF by adding a second opnSense and tie the two in failover.
So the setup would be:
- FTTC -> ISP1 router -> LAN cable 1 to port 1 of opnSense n.1
- FTTC -> ISP1 router -> LAN cable 2 to port 1 of opnSense n.2
- FWA -> ISP2 router -> LAN cable 1 to port 2 of opnSense n.1
- FWA -> ISP2 router -> LAN cable 2 to port 2 of opnSense n.2
Then in both opnSense i would setup failover multi-WAN and bridge them together so that one diyng will trigger the second one.
edit: fixed small errors
If that would be so simple... Are you a billionaire? In that case, it would make things MUCH easier.... :) Otherwise my wife might want a word....