How to set up Single NIC OPNsense (26.1) with 2x TP-Link Smart Switches & Fritz!

[Vectralis]

Hi everyone,

I am a complete beginner with OPNsense and networking in general. I want to build a "Router-on-a-stick" setup using a single-NIC PC with OPNsense 26.1, but I need some guidance on the exact step-by-step configuration.

My Goal & Hardware:
- ISP: Proximus VDSL (Belgium)
- Fritz!Box 7530 (currently acting as router/modem, want to use it purely as modem).
- OPNsense PC: Has only 1 physical NIC.

Switches:
- TL-SG105E (located in the Modem Room)
- TL-SG108E (located in the Router Room)

Target Network Settings:
- OPNsense LAN IP: 192.168.129.1/24
- Fritz!Box IP (for web management): 192.168.129.2/24

DHCP Range:
192.168.129.50 – 192.168.129.254

DNS: 1.1.1.1

Physical Connection Plan:

- Inter-switch link: Cable from TL-SG105E Port 1 to TL-SG108E Port 1.

- Modem Room (TL-SG105E):
Port 2 connected to Fritz!Box LAN 1 (for Internet traffic).
Port 3 connected to Fritz!Box LAN 2 (to access Fritz!Box web GUI at 192.168.129.2).
Ports 4–5 for local clients.

- Router Room (TL-SG108E):
Port 2 connected to the OPNsense single NIC.
Ports 3–8 for local clients / APs.

My Questions:

- VLAN Tagging / Switch Setup:
How should I precisely configure the 802.1Q Tagged/Untagged ports and PVIDs on both TP-Link switches for VLAN 1 (LAN) and VLAN 10 (WAN)?

- OPNsense 26.1 Setup:
Since I am using a single physical NIC:
What exact steps do I need to take in OPNsense to set up the VLAN 10 interface for WAN while keeping the physical NIC for LAN (192.168.129.1)?
How should I configure LAN / WAN settings?

- Fritz!Box Configuration:
Is connecting two cables from the Fritz!Box to the SG105E (one for WAN on VLAN 10, one for LAN management on VLAN 1) the correct way to keep access to 192.168.129.2?

Could someone walk me through the correct sequence of steps so I don't lock myself out during setup?

Thanks a lot for helping out a beginner!

And sorry if I ask dumb questions lol