If this is your first time using vlans, I recommend https://www.practicalnetworking.net/index/vlans-index/ which has a Cisco orientation, but covers the concept of vlans in an easy to understand way.
Ed Harmoush also has a Networking Fundamentals youtube playlist that is a good refresher for what switches and routers do.
Just remember, anything you can do with vlans can be done without them, it would just require more ethernet ports, switches, and wires. All that vlans do is allow you to share physical resources while emulating what could be done with additional hardware. Very similar in concept to what virtual machines do on a server, except for networks. Each vlan acts just like a LAN, everything on the same vlan has "direct access" to every other host on the LAN. A host can only be a member of more than one vlan if it has multiple virtual ethernet adapters (vlan interfaces).
You may find this thread useful as well Tutorial: Home VLAN configuration (RB5009, cAP ACs, multiple SSIDs), and the topics with the rtfum tag (odd choice for the tag name, but they are useful user threads).
As far as replacing ROS firewall, I agree with @Amm0; if your goal is to learn MikroTik, don't introduce something else. You already have the firewall in the Flint 2 (GL-MT6000) protecting the RB5009's "WAN" connection. The RB5009 should be getting its "WAN" ip address from the Flint 2's LAN dhcp server, which will be protecting the RB5000 from internet attacks.