I need a little help here
I want to implement this firewall rule
but i don't want it to affect my backbone links, only the links coming in through my AP, the issue is i have my client connect to the tower via PPPoE and somehow i need the rule to only affect those PPP interfaces and nothing else

Edit:

Okay, i've tried this but i haven't fully activate it yet.

2 chain=forward action=accept protocol=tcp in-interface=backbone1

3 chain=forward action=accept protocol=tcp out-interface=backbone1

4 chain=forward action=accept protocol=tcp in-interface=backbone2

5 chain=forward action=accept protocol=tcp out-interface=backbone2

6 X ;;; Allow up to 30 connections
chain=forward action=accept connection-state=new protocol=tcp
connection-limit=!30,32

as far as i understand the backbone links will hit their respective rules and then stop but all the other links will go past those until they hit the connection rule and stop there.

Am i correct in this thinking

Yes. with that configuration, only those tcp connections that their input or output interfaces are not backbone1 or backbone2, will reach rule number 6.
This will work assuming your pppoe users, are not connecting to the internet through backbone links.