IPv6 is a consideration simply because:
(a) it already exists in the OPs setup originating from one of the ISPs and
(b) the default behaviour of client OSes and applications effectively prefer IPv6 where available (or responds first)
, leading to BT being the primary connection more by accident than by conscious choice, and the OP wants to have more active control over the split.
In other words, even if the OP does nothing, it is already a factor in the routing.
btw in response to the original Q, DHCP isn't really intended distribute information about a second gateway on the same network.
The most that DHCP can do are options for classless static routes, which in effect provide advisory routes based on destination IP networks. In modern times this is less relevant when content is spread around CDNs because the option does not know anything about URLs, only IPs, and I say advisory because most client OSes now ignore this option. It was mainly a historical range of Windows OSes which implemented it.
The only routing information all clients will respect from DHCP is the single default gateway.
A dual-WAN router is still the way to go because all LAN traffic then passes through that point to make routing decisions without each client device needing to know about the multiple ISPs.
On that score, even though the OP already has a Pi, the architecture of how its network interface(s) attach to system buses is not suited to this application, though people do have some success using it for local DNS / filtering or proxy roles.
On the topic of disabling IPv6, there is a tendency to turn it off for the wrong reasons, such as not understanding it, and with a dual-WAN router, the routing decisions can be made dynamically based on latency, PING loss, throughput, protocol and a range of other factors without resorting to blunt / brute-forcing methods.
pfSense is indeed ideally suited to this as the features for WAN failover or balancing are of long-standing built-in rather than an afterthought, and fully managed from the web UI. Although people may find the (Netgate) appliance cost off-putting unless they have suitable spare hardware around, the hardware and licencing costs of more commercial offerings to enable equivalent features can rapidly outstrip the cost of pfSense over the lifetime of the boxes.
While Ubiquiti Edgerouters can also do this and at lower cost, having experience with both the latter has quirks especially if wanting to maintain the hardware offloading (acceleration) of flows used to achieve the headline throughput,, so you have to pay more attention to which features you combine, and know how to manage them from the CLI, where more advanced configurations are realised.
Draytek (Vigor) are ok-ish but returning to it to setup a new model recently was shocked to find so many basic changes still requiring a reboot with an outage; the UI and performance has evolved slowly compared to where it was 10+ years ago and most of the interfaces have fixed assignments (in terms of what can be mapped as a WAN or DMZ port vs the LAN ports).
IMO it is still targeting the "all-in-one-box solution" market and you have to pay close attention to the throughput specs when firewall/NAT/VPN are active compared with just routing for model selection.
prlzx on Zen: FTTC (VDSL) at ~40Mbps / 10Mbps
with IP4/6 (no v6? - not true Internet)
Edited by prlzx (Mon 08-Mar-21 02:09:11)