Hi,
yep, the Flint2 is more complex - that's because of OpenWrt and the way people want to use "basic" stuff which isn't basic at all ![]()
luci itself is supported - but it depends on what you are going to change. So, speaking in general, you should stick to the default GUI if you would like to get quick and fast support. If you don't care about the speed of support, you can modify settings in luci as well - but then people need to get deeper into your configuration to help you. luci is just a fancier version of editing config files via SSH.
Adding VLANs isn't supported by the GL GUI; so you need to go luci.
AdGuard Home and AdGuard DNS are entirely different services. On the device itself there is an installation of AdGuard Home where you can use block lists to block sites on connected devices - which upstream DNS server will be chosen is completely up to you. So yes, you can combine it with Cloudflare.
You don't need to be an "OpenWrt coder" but you need to understand what you are doing and how it will affect your device. So I am sorry at this point, but the official documentation is what you need - especially for VLAN. [OpenWrt Wiki] VLAN
Thread like Flint 2 GL-MT6000 - VLAN might help you as well.