If you don't care about entering home network from LTE side there is no real reason to get a NAT-less "dumb" LTE gateway, or an OpenWrt device.
If OPNSense had decent support for the "smart" LTE dongles that show up as USB ethernet dongles like Huawei ones with "hilink" in the name, I would recommend that, as you could skip having an additional device doing nothing more than physical interface bridge.
See this thread on how someone tried to deal with that https://forum.opnsense.org/index.php?topic=7872.0
I'm personally using and recommending a bare-bones LTE "router" with a single ethernet port, the Huawei B310
https://www.4gltemall.com/huawei-b310-lte-cpe.html (I'm linking this site only to show full info about the device and its variants for different locations in the world, which is very important as LTE frequencies that are OK for USA are NOT OK for EU or Asia)
Its web interface allows you to choose about NAT type (cone vs symmetric) and has port forwarding and DMZ settings, so you can do the "poor man bridge mode" if you want to.
It also has external antenna connectors but its internal antennas are good enough for me.
You can get used (branded) ones cheap on ebay for example, just make sure they are the right variant for your location, were unlocked and can be used with any LTE carrier.