Is that actually true? I thought "modem-mode" was removed but pass through is still there, so "all" one needs to do is not to configure the fritzbox itself with the correct PPPoE credentials (preferable no credentials at all) and click the "Angeschlossene Netzwerkgeräte dürfen zusätzlich ihre eigene Internetverbindung aufbauen (nicht empfohlen)". box... Which should result in working PPPoE passthrough. But that only helps if the OP actually uses PPPoE .
True, but if your ISP uses PPPoE you really do not need a true bridged-modem configuration, it should be enough to enable PPPoE pass trough while not configuring user and password for the router itself (this was reported as working for users of recent FritzBox's as of last year). You still will need to deal with potential VLAN issues.
I would probably try that first because:
a) newer DSL routers put emphasis on the VDSL2 part and the ADSL part is not that relevant anymore so sees few updates
b) you have that device already so testing should be quick and easy, once you have a new OpenWrt router... (such a setup also allows to place the "modem" as close to the socket (TAE?) as possible to reduce additional extension cords, and to place the router optimally for the WiFi coverage, you will need an ethernet cable between modem and router though 
)
But maybe you could tell us which ISP and country we are talking about here?