OK good news  

 It works !
I've configured as you suggest :
Port 14  Access Tagged for VLAN 11, Ingress checking Enabled
Port 15 and 16 Access Tagged for VLAN 10, Ingress checking Enabled
Port   eth15
VLAN Mode   Access
Access VLAN   11
Ingress Checking   Enable
Acceptable Frame Type   Untagged Only
Port 1 (the one connected to the OPNSense Router) configured like this:
Port   eth1
VLAN Mode   Hybrid
Native VLAN   1
Hybrid Untagged VLAN   1
Hybrid Tagged VLAN   10-11
Ingress Checking   Enable
Acceptable Frame Type   Admit All
Mr. Mom thank you, you're the King