Hi Bigcats,
Wife and I were in Bosnia last summer (2022) and there is no longer a need for a green card - they used to as you at the border crossings if you had a green card and if not you had to buy it - at the local bar/cafe etc.
I think it was altered when the EU changed their mind on green card requirements to try and get rid of post-Brexit problems around the Irish north-south border issues.
https://www.gov.uk/vehicle-insurance/driving-abroad
We've been in Bosnia a few times before and always had to buy insurance at border - but not any more
Last summer we entered Bosnia in far north at Velika Kladusa (south of Karlovac in Croatia) and travelled the length of the country and exited in the mountains due east of Trebinje into Montenegro.
We stayed in Jajce for 3 nights then Trebinje for 3 nights - as somebody else said, Trebinje is a hidden gem and we hope to return. Our accomodation in both towns was booked on Booking.com and both were abolutely superb and very reasonable and with secure bike parking.
It's a great area and Bosnia Herzogovina deserves far more attention from tourers, enjoy your trip.
Cheers.........................Grizzly