I believe your use of bridge(4) and vether(4) are needless complications. The vether(4) driver was developed to
solve a unique problem for one particular facility that required BGP peering.
Why not use your OpenBSD platform as a router? All you need is to enable packet forwarding.
{Internet} -- [ISP connected router] -- {Outer LAN} -- [OpenBSD router] -- {Inner LAN} -- [device]
If your ISP connected router provides DHCP services, those can be provided by
dhcpd(8).
If your ISP connection uses a standard interface, such as Ethernet, you could eliminate both the ISP connected router and the Outer LAN. Your OpenBSD platform becomes your gateway router to the Internet.
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Edited to add:
My home ISP is AT&T "U-Verse" service. The service uses an FTTN VDSL connection and requires an ISP-supplied gateway device. The gateway is used for VOIP and IPtv, which are on AT&T's private IP network. Internet services are routed through an inner OpenBSD router to the home LANs, in similar fashion to the diagram above.