Will all the APs be in range of each other and roaming likely to happen, or will they be suitably spread about enough that there are natural air-gaps and clients will likely not roam but disconnect and reconnect?
You might want to share a bit of a physical and logical topology diagram here to help us understand the deployment.
For now though I will assume the APs and BR2s will all be broadcasting in range of each other and potentially clients could easily end up moving between them, but the BR2s are all isolated from each other and form little islands paired with an AP each.
Whatever you do here is in reality a bit of a kludge, there is no key management between the different APs as far as I understand it (you may want to consider just making the network open with no encryption on that basis) so roaming will require the clients to perform a full authentication cycle if you use a PSK on the network.
The other issue you will have if there is no clean break is that most of the clients you are going to see here (smart phones and tablets I expect) will not likely realise they are on a different network as the ESSID is the same, so they will typically not attempt to renew their IP from DHCP, as such you’re going to need to try and make it look like internally they are the same thing, otherwise lets say you have different subnets on each BR2 a client could connect to one, then move to an AP that is connected behind a different BR2 and not realise - the client would still look like it is connected to WiFi but would have an IP from a different subnet and a gateway that doe not exist.
Configure the same SSID and security settings on your BR2s and APs, hopefully you are using Ic2 to manage all of this and that is very simple to do with just a few tags.
Configure a VLAN on each BR2 to handle the clients, make them the same subnet and make it suitably large that each router could handle the random associations of the client base.
Let’s keep this simple and say you have 200 expected users but they are going to potentially move about in such a way that each router/ap pair might see each client, I would configure a subnet large enough that each BR1 can have a /24 or so from within that subnet given out on DHCP, but with the remaining addresses still valid.
E.g. Covering prefix 10.1.0.0/20 (4096 addresses, or 16x /24 subnets)
Configure 10.1.0.1/20 on each BR2 as the IP on the VLAN you are going to use for the WiFi.
On BR2-1 configure DHCP to issue addresses from 10.1.1.1 - 10.1.1.254
On BR2-2 configure DHCP to issue addresses from 10.1.2.1 - 10.1.2.254
On BR2-3 configure DHCP to issue addresses from 10.1.3.1 - 10.1.3.254
This way no matter which bit of the network they join first the IP would still be valid, and as the DHCP scopes are separated there is less of a risk of a conflict - you may want to keep the lease time to around 1hr and make these pools bigger, but hopefully you get the intent here. A device that connects via BR2-1 and then moves to an area where the AP is connected to BR2-2 would probably not perform a DHCP renewal but would have a perfectly valid IP in the 10.1.0.0/20 subnet.
You are still going to have some problems though, as when clients move across the APs even though the same subnet is used and gateway the MAC address will be different, so they will have to ARP for who the new gateway is. Depending on the client that may or may not be a problem and the number of users (ARP is broadcast and you typically want to try and trim as much unnecessary broadcast and multicast traffic as possible over the air).
In Peplink (as far as I know) you cannot rewrite the LAN side MAC addresses, but we can make use the HA features to achieve the same result, configure all the BR2 as HA in the master role using the same group. Peplink HA uses VRRP at this point on the LAN side and will show the gateway as having the same virtual MAC address.
In terms of the WAN side you could look at configuring SpeedFusion on the BR2 but in all honesty for most applications you could just set the outbound policy to load balance the public across whatever WANs were available and for the most part that will work fine except for some applications (like wifi calling, or if people were say using Facetime or Whatsapp etc. or similar to make a call when they roamed).
After that there is probably a little bit of consideration required for how you configure the WiFi radios on the BR2 and APs in terms of setting them to sensible channels and power values to try and make who connects where a bit more deterministic.
In the US as far as Peplink are concerned you have 8x 20MHz channels in 5GHz available and you have 6 radios, I’d just statically nail them each to a different 20Mhz channel to start with and repeat a 1-6-11 plan on the 2.4GHz radios, power will depend on the required coverage area and proximity of the APs to each other, though in general I suggest running the 2.4GHz TX power one step lower than whatever you put the 5GHz at if using the simple “Max / Medium / Low” values.
For example:
BR2-1 ch36 & ch1/ AP-1 ch149 & ch6
BR2-1 ch40 & ch11 / AP-2 ch153 & ch1
BR2-3 ch44 & ch6 / AP-3 ch157 & ch11