You might want to look at this post that has an RB760iGS config (very similar to the RB750Gr3, but has an SFP port added). It has ether5 configured as trunk and was tested with a down stream switch successfully. Ether1 is WAN, ether2 is “off bridge access port”. Ports 3-5 are bridge ports. There are 3 vlans, 10, 15 and 20, plus an access port for vlan 10 on ether3 and and access port for vlan 20 on ether4 . The trunk is sending the 3 vlans tagged to the switch, and blocking non-tagged ethernet from the switch. There are 3 dhcp servers configured.
I can’t say much about the firewall in the config, I was concentrating on the bridge setup.
I would concentrate on getting the vlan things working first (behind your existing ISP router) and then after that is working, then work on the firewall. Making the firewall too tight during initial setup can prevent things from working (even if you have the bridge set up correctly, and you and easily spend a lot of time trying to fix problems that don’t exist, because the reason it isn’t working is because of a firewall issue (and don’t forget that windows firewalls can be significant as well, especially when mulitple subnets are involved).