In general due to the fact that you have subnets on different vlans they cannot talk at the L2 level, regardless if on different bridges or same bridge.
The firewall rule then controls what happens at the L3 level to ensure the router does or doesnt allow traffic in and out of the router as well as between subnets within the router.
Dont think you will need many changes if any, regarding inter router traffic.
Currently you have something like this:
/interface bridge
add name=bridge-vlan20
add name=bridge-vlan30
/interface vlan
add interface=ether1 name=e1v20 vlan-id=20
add interface=ether2 name=e2v20 vlan-id=20
add interface=ether1 name=e1v30 vlan-id=30
/interface bridge port
add bridge=bridge-vlan20 interface=e1v20
add bridge=bridge-vlan20 interface=e2v20
add bridge=bridge-vlan20 interface=ether4 #access port
add bridge=bridge-vlan30 interface=e1v30
add bridge=bridge-vlan30 interface=ether3 #access port
After you move to single bridge, you’ll have something like
/interface bridge
add name=bridge vlan-filtering=yes
/interface vlan
add interface=bridge name=bridge-vlan20 vlan-id=20
add interface=bridge name=bridge-vlan30 vlan-id=30
/interface bridge port
add bridge=bridge interface=ether1
add bridge=bridge interface=ether2
add bridge=bridge interface=ether3 pvid=30
add bridge=bridge interface=ether4 pvid=20
/interface bridge vlan
add bridge=bridge tagged=bridge,ether1,ether2 untagged=ether4 vlan-ids=20
add bridge=bridge tagged=bridge,ether1 untagged=ether3 vlan-ids=30
# untagged= is optional, it's implicitly set due to setting of pvid higher up
The rest will ve exactly the same in both cases (now and then) because you’ll have interface with same name which will allow router to interact with same VLAN. It doesn’t matter that currently you’re using interface implicitly created together with corresponding bridge while in future it’ll be a vlan interface anchored to common bridge. In a way: even though you’ll replace all the plumbing in the floors and walls, you’ll still have same watter taps with unchanged functionality.
So no need to rewrite any of L3 stuff (including firewall filter rules).