Hello Sorin
First of all, just a clarification. An Access Point can be in either Light-AP mode or FlexConnect mode, it cannot be both.
When you configure an AP to function in FlexConnect mode, you can either use an access port connection or a trunk port connection. It is not mandatory to use a trunk unless, as you say, it is serving multiple SSIDs. More info on this can be found here:
The same is true about a local mode AP. You can either use an access port or a trunk port. But in both local and FlexConnect cases, if a trunk is used, the access point needs IP connectivity on the native VLAN.
I hope this has been helpful!
Laz