OK good news
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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