As a WISP can I ask what is the advantages/ disadvantages of having PPPoE server at each tower when running
(1) OSPF
(2) L2 Bridged
(3) Both OSPF + L2 Bridged

Main advantage is spreading your risk to have a pppoe server at every pop. Second advantage is that not one router is doing all queues/firewall/natting for all clients.

Main advantage is spreading your risk to have a pppoe server at every pop. Second advantage is that not one router is doing all queues/firewall/natting for all clients.

Sounds good the advantage but by having the PPPoE at the tower does it mean that traffic must transverse the network to PPPoE server before Queue limits are applied?

Usually goes like this

Tower (various P2P and P2MP links) - switch - router (pppoe server / NAT / Queues / OSPF / Maybe some L2) - link back to your network.
You would then have a central radius server somewhere in your network for auth and IPs .