Starting with mesh networks

You'll need Level 2 bridging over the 802.11s links, as VLAN tagging is a Level 2 thing. GRE is one way to handle that.

as one reference. I use a slightly different configuration, eliminating one of the Linux bridges called out there, but the concept is the same. Performance of GRE is very good with low overhead and low CPU load. https://justus.berlin/2016/02/performance-of-tunneling-methods-in-openwrt/