Out of the box, pfSense can do multi WAN and CARP (similar to VRRP) clustering. At the office I have two older servers with lots of NICs and five WANs. Inbound redundancy is provided by dynamic DNS and SRV records etc. Note that to do CARP/VRRP, you do need at least a /29 IPv4 allocation. You need an address per box plus the virtual one that is actually used by services. PPPoA/E is harder to deal with than cable/leased line etc but it turns out that low cost Billion 8800NLR2 can do external IPv4 pass through as well as do the PPPoA/E. They will need an address as well from your range. You need something like them in this case because only one device can be the PPPoA/E dial up system at a time. Unless you have some very fancy secret sauce, your clustered routers' pppd or whatever are going to get confused as to who does what.
I notice you have a cloud key. Unifi on an Ubuntu VM is easy, and much easier to backup and snapshot before upgrades, so is safer. You can also front it with HA Proxy for simple URLs and perhaps Lets Encrypt. pfSense has a HA Proxy package with a GUI and I believe it is CARP friendly as well ...